Hells Belz (EDH / Commander)

Step 1: Cast Our Commander
First step is casting our commander. Since all 27 of our nonland cards in our deck have 4+ mana value, casting our commander will instantly draw our entire deck and deal a maximum of 27 damage to us. So our life total needs to be higher than that before casting Belzenlok or we need to give him lifelink with cards like Homicidal Seclusion or Deadly Wanderings. We also have ways to ramp out our commander a few turns early, mostly with lands like Peat Bog and Crystal Vein.

In order to combo off, we'll need either:

4BB if 1) an opponent controlling a Forest (or Forest dual) and you control a Swamp or 2) you control 5+ Swamps
OR

4BBB
OR

Deal combat damage to a player with Geode Golem to cast Demonlord Belzenlok for free.
If we can meet any of those three criteria, we can cast Demonlord Belzenlok, put all the nonland cards in our deck into our hand, and proceed to Step 2!

Step 2: Combo Win
Cast Demonlord Belzenlok, putting all the nonland cards from your library into your hand and taking that much damage (maximum 27).
Exile a non-combo card (anything not mentioned here) to cast Unmask on yourself and discard Phantasmagorian.
Activate Phantasmagorian, discarding Skirge Familiar, Dread Return, and a non-combo card to return Phantasmagorian to your hand.
Now you need a third creature on the battlefield. Cast Deepwood Legate for free if any opponent controls a Forest (or Forest dual), or cast Dross Golem for free if you control 5+ Swamps. If you can't do either and have no third creature already then you'll need an extra B: you'll need to activate Phantasmagorian two additional times (holding priority) to discard 6 non-combo cards to delve Sibsig Muckdraggers for B, returning a non-combo creature to hand.
Cast Hollow One for free.
Flashback Dread Return, sacrificing Demonlord Belzenlok, Hollow One, and your third creature (Deepwood Legate / Dross Golem / Sibsig Muckdraggers) to reanimate Skirge Familiar.
Use Skirge Familiar's ability to discard Cackling Imp, Grim Poppet, Cinderhaze Wretch, Skithiryx, the Blight Dragon, and a non-combo card for BBBBB.
Spend BBBB to cast Necrotic Ooze.
Spend B to give Necrotic Ooze haste from Skithiryx, the Blight Dragon.
Tap Necrotic Ooze to make an opponent lose 1 life (from Cackling Imp).
Put a -1/-1 counter on Necrotic Ooze to untap it (from Cinderhaze Wretch).
Target any creature other than Necrotic Ooze, remove the -1/-1 counter from Necrotic Ooze as a cost. Then, before the "Put a -1/-1 counter on another target creature" resolves hold priority (from Grim Poppet).
With the "Put a -1/-1 counter on another target creature" on the stack, repeat steps 11-13 to make your opponents lose infinite life and win the game.