Dementia Bat — 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.

Dementia Bat — ruling 2

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

Dementia Bat : règles, fonctionnement et interactions (MTG)

Dementia Bat 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

Flying , Sacrifice this creature: Target player discards two cards.

How it works : Dementia Bat

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 Dementia Bat’s ability in response. The spell is on the stack by then.
No other card is named in this ruling.