Basri's Lieutenant + Metallic Mimic + Phyrexian Altar

Basri's Lieutenant

Metallic Mimic

Phyrexian Altar

How it works

1. Activate Phyrexian Altar by sacrificing another creature with a +1/+1 counter on it, adding one mana of any color.
2. When the creature dies, Basri's Lieutenant's second ability triggers, creating a 2/2 Knight creature token that enters the battlefield with a +1/+1 counter due to Metallic Mimic.
3. Repeat.

This loop turns the recursive lifecycle of your army into an endless engine of colored mana. With Metallic Mimic set to choose Knights, any incoming Knight naturally enters the battlefield pre-loaded with a +1/+1 counter. You can then feed that creature to Phyrexian Altar to generate mana, triggering Basri's Lieutenant because the sacrificed creature possessed a counter when it died. That death trigger immediately replaces the fallen warrior with a fresh 2/2 Knight token, which the Metallic Mimic immediately beefs up with another counter so the entire sequence can begin anew.