Cheapest priced copy (Commander Legends: Battle for Baldur's Gate): $4.99 at Card Kingdom, near mint, their price list as of .
Death Kiss 
Creature — Beholder · 5/5 · Rare · Commander Legends: Battle for Baldur's Gate · #675
Whenever a creature an opponent controls attacks one of your opponents, double its power until end of turn.
{X}{X}{R}: Monstrosity X. (If this creature isn't monstrous, put X +1/+1 counters on it and it becomes monstrous.)
When this creature becomes monstrous, goad up to X target creatures your opponents control.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $4.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Legends: Battle for Baldur's Gate | 629 | Rare | $4.99 | — |
| Commander Legends: Battle for Baldur's Gate | 675 | Rare | $4.99 | — |
4 rulings from Wizards of the Coast
- 10 June 2022To double a creature's power, it gets +X/+0, where X is its power as that ability resolves.
- 10 June 2022Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
- 10 June 2022Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
- 10 June 2022An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.