Activate Darkheart Sliver by sacrificing it.
When Darkheart Sliver dies, Mortuary triggers, putting Darkheart Sliver on top of your library from your graveyard.
Resolve the Darkheart Sliver ability, causing you to gain 3 life.
Cast Darkheart Sliver from the top of your library by paying 2 life.
Repeat.

Infinite death triggers

Infinite ETB

Infinite lifegain

Infinite lifegain triggers

Infinite LTB

Infinite sacrifice triggers

Infinite storm count

Mortuary

Darkheart Sliver

Bolas's Citadel

Activate Gravecrawler by sacrificing itself, causing you to gain three life.
Prize Pig triggers, putting three ribbon counters on it, then removing all ribbon counters on it and untapping it.
Activate Prize Pig by tapping it, adding {B}.
Cast Gravecrawler from your graveyard by paying {B}.
Repeat.

Combos with "Darkheart Sliver"