Altar of Shadows — ruling 1

You put the counter on Altar of Shadows even if the creature regenerates.

Altar of Shadows — ruling 2

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.

Altar of Shadows : règles, fonctionnement et interactions (MTG)

Altar of Shadows is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

At the beginning of your first main phase, add for each charge counter on this artifact. , : Destroy target creature. Then put a charge counter on this artifact.

How it works : Altar of Shadows

Original English text — Wizards of the Coast / Scryfall

#1December 1, 2004· Wizards of the Coast
You put the counter on Altar of Shadows even if the creature regenerates.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
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.
No other card is named in this ruling.