This is possibly so simple that it would border on boring, but what the heck, [scryfall]Terminus[/scryfall] isn’t as popular as it used to be due to the new tucking rules, maybe this could do something.

What do we need? Sweepers and ETB-creatures you can sac to Yahenni. Your friends are going to love you.

Sweepers:

All of that would completely destroy the curve on this deck, so maybe not go quite this far.

Creatures with ETB-abilities:

Sadly, these abilities are pretty one-note in black, which also means the curve will get worse yet again.

Gladly, we have cheap creatures that can self-recur, meaning that they are good fodder for Yahenni:

Maybe [scryfall]Gravecrawler[/scryfall] as well, but I’m not sure how many zombies we’ll have.

Other recursion effects and continuous sources of creatures:

… and finally, to keep the curve low, at least a bit, a bunch of cheap, effective removal:

Oh yeah, if things are going to die all the time, we might as well try to benefit from that.

Some cardflow would help as well, so I’ll put in a few of those. There are some already in there, but a few more would be appreciated.

That’ll do. Not the most effective set, but there we are.

Since the curve is pretty high, I’ll put in some manarocks. I’m not the biggest fan of ramping in general, but in this case we just need it. I’ll even put in a [scryfall]Sol Ring[/scryfall], which I hardly ever do.

… 40 lands, which means we’ll have to make quite a few cuts before the final list.

So, the final list would look like this:

[deck]
[commander]
Yahenni, Undying Partisan
[/commander]
[creatures]
Big Game Hunter
Blood Artist
Bloodghast
Bloodsoaked Champion
Bone Shredder
Brood of Cockroaches
Deathbringer Regent
Endless Cockroaches
Falkenrath Noble
Fleshbag Marauder
Gatekeeper of Malakir
Gilt-Leaf Winnower
Gonti, Lord of Luxury
Grave Titan
Gray Merchant of Asphodel
Harvester of Souls
Kothophed, Soul Hoarder
Liliana’s Specter
Massacre Wurm
Merciless Executioner
Nekrataal
Nether Traitor
Noxious Gearhulk
Ophiomancer
Overseer of the Damned
Puppeteer Clique
Reassembling Skeleton
Sepulchral Primordial
Shriekmaw
Slum Reaper
Smothering Abomination
Zulaport Cutthroat
[/creatures]
[spells]
Corpse Dance
Eyeblight’s Ending
Go for the Throat
Hero’s Demise
Hero’s Downfall
Murder
Rend Flesh
Terror
Ultimate Price
Crux of Fate
Damnation
Decree of Pain
In Garruk’s Wake
Life’s Finale
Necromantic Selection
Read the Bones
Armillary Sphere
Everflowing Chalice
Hedron Archive
Mind Stone
Oblivion Stone
Sol Ring
Thran Dynamo
Wayfarer’s Bauble
Whip of Erebos
Bitterblossom
Phyrexian Arena
[/spells]
[lands]
Cabal Coffers
37 Swamp
Urborg, Tomb of Yawgmoth
Volrath’s Stronghold
[/lands]
[/deck]