Argentum Masticore
Artifact Creature — Phyrexian Masticore
First strike, protection from multicolored
At the beginning of your upkeep, sacrifice this creature unless you discard a card. When you discard a card this way, destroy target nonland permanent an opponent controls with mana value less than or equal to the mana value of the discarded card.
Zack Stella
Rulings
Notably, this means that you may discard a card to keep Argentum Masticore on the battlefield even if there isn't a legal target for that second triggered ability. In that case, no players may respond to the second triggered ability and it does not resolve.
(wotc • 2023-02-04)
You don't choose a target for Argentum Masticore's upkeep triggered ability at the time it triggers. Rather, a second "reflexive" ability triggers when you discard a card this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.
(wotc • 2023-02-04)
Rulings
Notably, this means that you may discard a card to keep Argentum Masticore on the battlefield even if there isn't a legal target for that second triggered ability. In that case, no players may respond to the second triggered ability and it does not resolve.
(wotc • 2023-02-04)
You don't choose a target for Argentum Masticore's upkeep triggered ability at the time it triggers. Rather, a second "reflexive" ability triggers when you discard a card this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.
(wotc • 2023-02-04)