Sidisi, Undead Vizier + Helm of the Host + Nexus of Fate

Sidisi, Undead Vizier

Helm of the Host

Nexus of Fate

How it works

1. At the beginning of combat on your turn, Helm of the Host triggers, creating a non-legendary copy of Sidisi with haste.
2. The non-legendary Sidisi enters the battlefield, triggering its exploit ability, sacrificing itself.
3. Sidisi's second ability triggers, allowing you to search your library for Nexus of Fate, put it into your hand, and shuffle your library.
4. Cast Nexus of Fate by paying 5, causing you to take an extra turn after this one and shuffle it into your library.
5. Repeat.