Birds of Paradise, Elves of Deep Shadow, Elvish Mystic, Fyndhorn Elves and Llanowar Elves are all 1-cost mana dorks that accelerate our early game by allowing us to turbo out Meren and our other spells ahead of curve. They also serve as efficient sac fodder because we only need 1 Exp counter for Meren of Clan Nel Toth to revive them.
Spore Frog can be sacrificed at instant speed to fog any combat damage that would be dealt to us and because it's so cheap to cast, it's very easy to recur with Meren's effect, making it very difficult for opponents to deal combat damage to us.
Haywire Mite is efficient removal for Artifacts and Enchantments and does everything we want in this deck. It sends itself to graveyard which gives us an Exp counter with Meren and because it's just 1 CMC it's pretty much always available for Meren to revive so we can keep Artifact and Enchantment decks in check, all while gaining more experience counters.
Carrion Feeder and Viscera Seer are in here for the same reasons. They cost 1 mana to cast, they can sacrifice creatures (including themselves) at instant speed and they enable our combos. These are also some of the most efficient ways to generate experience counters because if we have Meren of Clan Nel Toth out, we can just sac them to themselves, get an exp counter and then just bring them back at the end of turn.
Disciple of the Vault is literally just a combo piece and part of a Protean Hulk package that can win the game on the spot.
Lesser Masticore is another piece of that same combo. The Persist makes it good sac fodder because it just brings itself back ready to be sac'd again.
Melira, Sylvok Outcast prevents us from getting Poison counters which is useful against the rogue Infect cards we might encounter but it's mainly in here to enable the combo of Disciple of the Vault + Lesser Masticore + a repeatable sac outlet. It prevents -1/-1 counters from being placed on our creatures which means that Lesser Masticore reviving itself with Persist will be at normal power/toughness and can be repeatedly sacrificed.
Fauna Shaman helps our strategy by dumping creatures into our graveyard in exchange for any creature in our deck. With Meren of Clan Nel Toth on the board, we can simply retrieve the creature we discarded or just reanimate it if we have enough experience counters.
Dauthi Voidwalker is fantastic in this deck for a couple of reasons. Firstly, it forces our opponents to exile any cards that would otherwise end up in their graveyards with a Void Counter on them. So with just that effect alone, it disrupts several decks reliant on their graveyard. Then, the Voidwalker can sac itself and let us cast one of those cards for free. That alone is pretty good but then realize that we can recur this repeatedly with Meren so we can keep disrupting our opponents' graveyard strategies and repeatedly steal their stuff.
Fiend Artisan is sort of like a Green Sun's Zenith except it requires a sacrifice and can grab any creature with CMC equal to the amount we spent on X. So a death trigger for Meren and any creature we want from our deck.
Sakura-Tribe Elder is fantastic value in this deck because it kills itself to ramp which gives us a Meren experience counter and due to its 2 CMC, it can very easily be revived with Meren to do it all over again.
Farhaven Elf lets us grab any basic land out of our deck when it ETBs. Obviously great to reanimate if there's nothing better to do so we can ramp even more. We're playing this instead of Wood Elves because Wood Elves can only fetch Forests, whereas Farhaven Elf can get either Forests or Swamps, and in this deck we have some pretty heavily-colored casting costs so having that option of either a Swamp or Forest is important in this particular build.
Eternal Witness provides some nice utility of being able to Regrowth any card from our graveyard when it ETBs. Meren of Clan Nel Toth can only reanimate and retrieve creatures, so having the option of recycling an instant, sorcery, enchantment or artifact is just nice to have.
Fleshbag Marauder, Merciless Executioner, Demon's Disciple and Plaguecrafter all serve the same function. When they ETB they force everyone to sacrifice a creature. For us, we can just sac it to itself, thus triggering Meren's effect to give us an experience counter and forcing all opponents to get rid of one of their creatures. Finally, if we have 3 EXP counters we can just bring one of these back at the end of the turn and do it again. These help police the board and keep our opponents from advancing their board state.
Gravelighter is sort of in the same club, however it works a little differently depending on whether or not a creature died. If one did, Gravelighter draws us a card when it ETBs. If one didn't, it forces everyone to sac a creature just like Fleshbag Marauder. Either way, it's a win-win for us.
Grim Haruspex rewards us for all the sacrificing we're doing by drawing us cards. Not a whole lot else to be said here, it's just great value in an aristrocrats-style deck.
Morbid Opportunist is in here for the same reason as Grim Haruspex, reliable card draw in a deck focused on self-sacrifice (although not quite as good due to only triggering once per turn).
Braids, Arisen Nightmare is incredible in this deck. On our end step, we have the option to sacrifice one of our permanents and then all opponents have the choice to sacrifice one of the same type. If they don't we draw a card. Either way, it's a win-win for us. We get a death trigger for Meren (if we picked a creature), and we get rid of our opponents' stuff OR we get to draw cards. And recall that Braids can sac herself, so if we have Meren with at least 2 Exp counters, we can sac Braids, get another Exp counter and just revive her with Meren's effect.
Woe Strider mainly in here because we want to be playing as many instant-speed sac outlets available to us. The nice thing is that it brings its own Goat token with it to get us started with the death triggers.
Yahenni, Undying Partisan is in here for the same reason, except it's a little more useful because it can make itself indestructible at instant speed while giving us a death trigger.
Yawgmoth, Thran Physician is another instant-speed sac outlet except this one also requires us to pay 1 life. The trade-off is that we get to cripple a creature and draw a card when all is said and done.
Vile Entomber is basically a repeatable Entomb in this deck because we can dump something with the ETB trigger, then sacrifice Vile Entomber to one of several sac outlets and just reanimate it with Meren later to do it all over again.
Ravenous Chupacabra straight up destroys an enemy creature when it ETBs. Just a useful way to control the board and answer threats and recurrable with Meren if we have at least 4 Exp counters.
Phyrexian Delver is mainly in here because it enables a secondary combo with Protean Hulk, but we can also use it as a reanimation effect if we need to.
Sidisi, Undead Vizier does everything we want in this deck. It forces one of our creatures to die which will trigger Meren's effect, and then we get to Demonic Tutor for any card we want.
Mikaeus, the Unhallowed is mainly in here for combo purposes but it provides a blanket buff to most of our creatures and gives them Undying which means they'll be coming back whenever they die or we sacrifice them for our own effects.
Wurmcoil Engine helps keep us healthy with its Lifelink but the fact that we can just kill with a sac outlet to get 2 tokens and then just reanimate it with Meren and repeat just makes it that much better.
Protean Hulk is our main combo enabler. Not much to say here, get it on the board and find a way to kill it, then we get to combo off. See the Combos section for more details.
Butcher of Malakir is basically Grave Pact on a body. We're doing a lot of self-sacrificing so our creatures are going to be dying left and right. The more of these edict effects we have on the field, the more we benefit from our creatures dying. And since Butcher of Malakir is a creature, it can be reanimated or retrieved with Meren, making it very oppressive in this deck.