Mortuary + Phyrexian Altar + Bolas's Citadel + Tarpan

Mortuary

Phyrexian Altar

Bolas's Citadel

Tarpan

How it works

1. Activate Phyrexian Altar by sacrificing Tarpan.
2. When Tarpan dies, it and Mortuary triggers.
3. Resolve the Mortuary trigger, putting Tarpan from your graveyard onto the top of your library.
4. Resolve the Tarpan trigger, causing you to gain 1 life
5. Cast Tarpan from the top of your library by paying 1 life due to Bolas's Citadel.
6. Repeat.

This clever loop harnesses Bolas's Citadel to cast spells directly from the top of your library by trading life instead of mana, allowing Tarpan to be repeatedly played right from the top. When you sacrifice Tarpan to Phyrexian Altar, you generate a mana of any color while triggering its death ability to offset the life paid. Meanwhile, Mortuary intercepts Tarpan as it enters the graveyard, neatly placing it back on top of your library so the cycle can start anew.