({3} magic symbol) ({B} magic symbol) ({R} magic symbol) available if Garna is in the command zone plus enough mana to pay for commander tax, if applicable.
({R} magic symbol) if Garna is in hand available.
Steps
If Garna is in the command zone, cast it by paying ({3} magic symbol) ({B} magic symbol) ({R} magic symbol) and commander tax, if applicable.
Otherwise, activate Sneak Attack by paying ({R} magic symbol) , putting Garna from your hand onto the battlefield.
Garna's enter the battlefield ability triggers.
Holding priority, activate Phyrexian Altar by sacrificing Gray Merchant and Garna, adding ({R} magic symbol) ({R} magic symbol) .
Resolve the Garna ability, returning all creature cards put in your graveyard this turn to your hand.
Activate Sneak Attack by paying ({R} magic symbol) , putting Gray Merchant from your hand onto the battlefield.
Gray Merchant enters the battlefield, causing each opponent to lose life equal to your devotion to black, and causing you to gain life equal to the life lost this way.
Activate Sneak Attack by paying ({R} magic symbol) , putting Garna from your hand onto the battlefield.