Miasma Demon — ruling 1

You don't choose targets for Miasma Demon's last ability at the time it triggers. Rather, a second "reflexive" ability triggers when you discard cards this way. You choose targets for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.

Miasma Demon : règles, fonctionnement et interactions (MTG)

Miasma Demon 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 When this creature enters, you may discard any number of cards. When you do, up to that many target creatures each get -2/-2 until end of turn.

How it works : Miasma Demon

Original English text — Wizards of the Coast / Scryfall

#1September 20, 2024· Wizards of the Coast
You don't choose targets for Miasma Demon's last ability at the time it triggers. Rather, a second "reflexive" ability triggers when you discard cards this way. You choose targets for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.
No other card is named in this ruling.