Shrine of Limitless Power — ruling 1

Unlike most abilities that force a player to discard cards, this ability may be activated whenever you could cast an instant, including during your opponent’s draw step after they have drawn a card.

Shrine of Limitless Power — ruling 2

You can’t force an opponent to discard a card they cast by activating the last ability in response. The spell is on the stack by then.

Shrine of Limitless Power : règles, fonctionnement et interactions (MTG)

Shrine of Limitless Power 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 upkeep and whenever you cast a black spell, put a charge counter on this artifact. , , Sacrifice this artifact: Target player discards a card for each charge counter on this artifact.

How it works : Shrine of Limitless Power

Original English text — Wizards of the Coast / Scryfall

#1June 1, 2011· Wizards of the Coast
Unlike most abilities that force a player to discard cards, this ability may be activated whenever you could cast an instant, including during your opponent’s draw step after they have drawn a card.
No other card is named in this ruling.
#2June 1, 2011· Wizards of the Coast
You can’t force an opponent to discard a card they cast by activating the last ability in response. The spell is on the stack by then.
No other card is named in this ruling.