Felidar Guardian, Vesperlark, and Altar of Dementia

Prerequisites

• Felidar Guardian in hand
• Vesperlark on the battlefield
• Altar of Dementia on the battlefield
• available

Steps

1. Cast Felidar Guardian by paying
2. When Felidar Guardian enters the battlefield, it triggers
3. In response, activate Altar of Dementia by sacrificing Felidar Guardian, causing target player to mill one card
4. Resolve Felidar Guardian's ability, blinking Vesperlark
5. When Vesperlark leaves the battlefield, its ability triggers, returning Felidar Guardian from your graveyard to the battlefield
6. Repeat from step 2

Results

• Infinite ETB
• Infinite LTB
• Infinite mill
• Infinite sacrifice triggers