Adarkar Valkyrie + Hyrax Tower Scout + Phyrexian Altar

Adarkar Valkyrie

Hyrax Tower Scout

Phyrexian Altar

How it works

1. Cast Hyrax Tower Scout with its enter the battlefield trigger targeting Adarkar Valkyrie.
2. Maintaing priority, anctivate Adarkar Valkyrie's ability targeting Hyrax Tower Scout.
3. Adarkar Valkyrie's ability resolves and Hyrax Tower Scout's trigger resolves untapping Adarkar Valkyrie.
4. Sacrifice Hyrax Tower Scout to Phyrexian Altar.
5. Adarkar Valkyrie's ability triggers and returns Hyrax Tower Scout to the battlefield.
6. Repeat.