There are two ways to go about building combos or synergies in cube - Either you go all in and build your entire deck around that idea, or you strip it back and find the best existing thing with good overlap to pair it with. In constructed, it is almost always best to go all in on your single plan and build around it, but in cube it can often be better to combine things due to the lack of depth and redundancy in the narrower themes. Obviously, we only have one Quest for the Holy Relic to work with and that is our first and main limiting factor. We can load up on tutors to get it, but as a result, we become slower and more vulnerable to disruption. Certainly, if you build around Quest you can fire it off turn two and have some pretty unbeatable starts. Sadly, I think that is unlikely happen often enough to offset the other issues the deck would have. I think you could make it serviceable, but getting a pure Quest deck to tier one sounds impossible. Luckily,there are some conveniently good-looking places where it is fairly easy to insert a concise Quest package of two cards.
This list is a mashup of three archetypes, which is about as far as you can push the mashups. Even this would serve the individual components a lot better by losing the lifegain elements, I suspect. It might intuitively sound better to have two combos running at 90% rather than three at 70% or whatever, but when those game planes are sufficiently different that they cover a broader portion of the meta effectively, then it is worth it. Each of the different elements of this deck, the Quest, the Monument and the Soul stuff have some pretty strong matchups. The overlap in cards between the three parts is sufficient that the cost of combining them is fairly minimal too. That is not too surprising, given that the best way to build a Soul Sisters deck does seem to be with Oketra's Monument. You could just think of this as a mashup of two decks due to that, but it is more semantics at that point.
Here is the list;
Quest for the Holy Relic
Wall of Omens
Recruiter of the Guard
Ranger of Eos
Archangel of Thune
Return to the Ranks
The first game plan is to make Quest, then make five guys and ensure you have one in play, ready to attack. Do that fast enough and you beat most things. It is still a powerful swing at any later point in the game, too. You can improve this aspect of the deck with more creatures, cheaper creatures and ones that are hardier or that protect others.
The second game plan is to make Monument and then churn out an unreasonable number of 1/1s, in addition to doing other things faster than usual as well. Ideally, you just use value creatures to find more dorks to make. Bugler into Recruiter into Inspector, etc. Failing that, you can just repeatedly cast a self bounce dork and make as many 1/1s per turn as you have white mana. Most decks fold to such things eventually.
The last game plan is to gain loads of life with Soul Warden effects and apply that to some payoff. Usually, you don't need the payoff, because this strategy is best when it hard counters aggression or combos through the life gain itself. It is nice to have a few efficient dorks and a few alternate win conditions too, of course. This works well with Monument, as it is producing double dorks thus double the life gain and because it has dorks you can cast repeatedly. It is lovely when you setup such that a white mana produces four life and a 1/1 endlessly!
Being fairly close to a white weenie deck, you also have the fourth plan of simply beating down with random janky dorks. Soul Sisters is usually pretty good at that with some Anthem effects on the go, but those were mostly the cards culled to fit in the other synergies. As such, this is only really offering the aggressive plan against decks without much in the way of combat creatures. The lack of removal in this list makes it doubly hard to get into combat when the opposition has dorks. It isn’t too much of a problem, as you gain so much life it is hard to safely get damage in that actually threatens anything. That to do so would involve attacking with everything and leaving back no blockers ensures you are losing the race to this list.
Being a deck with three sub-themes, there is a whole lot of extra options that are really good for this list and each of which will empower aspects of the list to work better. That ability to balance which sub-themes work best allows for some lovely fine tuning potentials for any given meta, which is a quality I like in a deck.
With the dig and tutor effects on offer, I quite wanted to include some utility cards such as Warpriest of Thune, Fiend Hunter, Kor Sanctifiers, Restoration Specialist, Knight of the White Orchid or even just Oreskos Explorer for the easier cast. Much as such things are cute, there isn't enough room. The deck is already dangerously light on removal, and I think I would better off starting with a Council's Judgement or Cast Out than these sort of things.
I went light on interactive spells, as I often do in deck testing, but this kind of build is absolutely the best for getting away with such things. There are very few cards this deck needs to remove. If it faces a card it cannot remove or beat with the plan at hand, you at least have several other plans you can try instead. One is usually able to get there.
Banisher Priest is likely the best creature option for utility overall, as you can find it with both Recruiter and Bugler. It cannot be abused like Fiend Hunter with your Whitemane Lion for the permanent exile, but that is a lot more corner case than ease of tutoring. Much as having a removal option on a dork is a nicely condensed way of making the deck more robust, I don't like the Banisher Priest style of card sufficiently to want to play it without more support. Ideally both Mother of Runes and Dauntless Bodyguard, in addition to the current Selfless Spirit.
I quite like the idea of lowering the curve in general, as any time you can do that without lowering the effectiveness of the deck you are gaining a great deal. Both Mardu Woe Reaper and Soldier of the Pantheon offer a good amount of early pressure, while also having some synergy with the life gain elements. While adding in these kinds of cards is quite good generally, it isn't as simple as that. It really doesn't take long in this list before you tip the balance and really should be playing anthem effects. Benalish Marshall somewhat supports all three components of the deck, at least compared to Honor the Pure. Ajani Goldmane only supports the two, but he does offer an alternate angle of attack. Mother of Runes should probably be in regardless, as it has at least mild synergy with your various game plans, good synergy with the Quest and such a high baseline power level for such a cheap card.
As for where you might make space in the list for such fancies, you could replace the Legion's Landing which had poor synergy with the Quest and the Bugler. I like what Landing brings to the deck and some of the other synergies it helps, but it is certainly one of the more polar cards. It also wouldn't lower the curve doing that.
I could easily see cutting Suture Priest. While having lots of Soul Warden cards is nice, they are seemingly the least important bit of the deck. I tried out Ajani's Welcome, but it was a wasted card as it had just the one synergy it supported. Welcome is only for the dedicated life gain decks. Auriok Champion is great, despite the inability to get a cost reduction, because it is pretty resilient to removal and thus a great thing to slap Argentum Armor on. This difference in overlap makes Champion perfect and Priest a possible cut, despite how similar they are in many other ways.
Other cuts are possible in some of the clunkier support tools. Wall of Omens is great, but it is the opposite of something that applies pressure. It supports Quest less well than most of the other creatures in the list, supports the life gain in a fairly minimal way, the aggro options appallingly and only the Monument actually well. A bit like Landing, it is very polar in how it aids the deck. Despite looking off theme, being a cheap cycle card probably makes it too good to miss, especially in white. It just makes for a smaller deck and thus better chances of having the synergy overlaps you want. Bugler and Recruiter are also clunky and fall into a similar sort of camp. Bugler does best in tempo terms, but has some reliability issues and needs considering in the build,which might make him more effort than he is worth.
The Aviary Mechanic might also be a touch deep as you want more than two such bounce cards, but you probably don't need the full three. Sadly, I am already running all the sensible dig and tutoring white has to offer and so there is no way to buffer that cut. While it is exactly what you want should you have a Monument in play, it is pretty weak when you don't. Monument is good enough on its own that I think three bounce cards is perhaps too much of a win more situation. Stonecloaker is a better card that you can use, but it is not better with Monument specifically, as it isn't reduced in cost to one mana.
Other cute bounce options are Glinthawk plus Ornithopter, Memnite, Phyrexian Walker and however deep down that hole you care to go. This is used to good effect in constructed, but I think this is too much weight on the Quest completing speed side of the deck and will lead to far too many dud draws. Glinthawk does not have that perpetual self-bounce effect you most desire for the Monument draws, either.
Leonin Vanguard worked out nicely. It is my first use of the card and I found it pleasing in this build. It does a bit of everything at the right price. A creature trigger, a target for all the dig/tutors, a lifegain trigger, a cheap card, a decent attacker, a good thing to draw with using Skullclamp and all that sort of thing. It just has a solid power level and has no lumpy interactions with the deck as a whole, like Wall of Omens or Legion's Landing. Blade Splicer and Flickerwisp were both considerations for their various synergies with the other bounce and EtB effect cards in the deck, too. Both add reasonable punch and power on their own, but what they offer the various synergies is a bit mild for the weight of these cards.
Some of the best payoff for lifegain in cube is top end. I only managed to find room for Archangel of Thune in the list with Felidar Sovereign and Crested Sunmare relegated to the bench. Sovereign is amazing as it provides an actual I win card that gives you some decent inevitability in the lifegain lines. At six mana, however, it is really hard to build a deck than can cast it in the time frame needed. It leads to some pretty bad draws, which the Argentum Armor is already doing! Sunmare is just a different sort of Archangel, powerful but slow, and onerous on the build.
Lastly, we have some of the recursion options. I think I was just being a bit precious in running Return to the Ranks. I felt like I could have certain strategies nullified too easily with some well placed removal. I wanted to be able to get my lifegain cards back against aggressive players, my high value threats like Ascendant back against slower decks and my bounce cards back whenever they became useful! With this deck having so many angles of attack and a reasonable ability to maintain gas, you can probably just forget about dead cards and move on to greener pastures. Return to the Ranks was very powerful, but it mostly sat in my hand collecting dust. I didn't need it and the luxury of it was perhaps not worth the cost of the bad hands and weaker starts. Despite this, I had considered many options in its place! Dusk // Dawn is very powerful and offers extra utility and early game action on Return. It is likely the best replacement, but sadly it is yet another card that has mixed interactions. I would feel happier about running it if it didn’t kill my own things! Proclamation of Rebirth is more a Martyr of Sands affair, and is pretty limited in a dedicated Soul Sisters deck. In this list it would be far too limited. Emeria, the Sky Ruin probably has the least negative impact on a build of the recursion elements! That, or even Mistveil Plains.
Overall, the deck performed well. The Monument side of the deck was the most powerful, but everything contributed. The list was less vulnerable to mass removal or running out of gas than previous iterations of Soul Sister's decks. While the maindeck felt very powerful, I would be fearful of struggling much more with Disenchant/Naturalize effects after side boarding. All your key synergy cards are taken down with such effects and render your deck a very low powered blend of random white dorks! This is where having a slightly better aggressive plan comes in really handy! The namesake of the deck performed well. Technically you could make it turn one and be attacking with Argentum Armour on turn three, but that requires all the one drops. It was much more frequently coming out turn four or five when you lead with the Quest. This wasn't as aggressive and didn't have that lockout of the game potential that killing early land drops has, but it still did huge amounts of work. The Quest and Armour are such a neat little package I expect to be using them again in a wide selection of places, some of which I am sure will perform better than in this list. That being said, this list remains one of the solid obvious homes for Quest for the Holy Relic.