At the beginning of your precombat main phase, addto your mana pool for each charge counter on Altar of Shadows.
, : Destroy target creature. Then put a charge counter on Altar of Shadows.
10/4/2004 The precombat main phase is the first main phase of the turn. All others are postcombat main phases, even if they technically occur before combat.
12/1/2004 You put the counter on Altar of Shadows even if the creature regenerates.