At the beginning of your precombat main phase, add {B} for each charge counter on Altar of Shadows.
{7}, {T}: Destroy target creature. Then put a charge counter on Altar of Shadows.
You put the counter on Altar of Shadows even if the creature regenerates.
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.