Cheapest priced copy (Warhammer 40,000 Commander): $0.35 at Card Kingdom, near mint, their price list as of .
Trygon Prime 
Creature — Tyranid · 4/4 · Uncommon · Warhammer 40,000 Commander · #143
Subterranean Assault — Whenever this creature attacks, put a +1/+1 counter on it and a +1/+1 counter on up to one other target attacking creature. That creature can't be blocked this turn.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.35 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Warhammer 40,000 Commander | 143 | Uncommon | $0.35 | — |
| Warhammer 40,000 Commander | 143★ | Uncommon | — | — |
One ruling from Wizards of the Coast
- 7 October 2022"That creature" refers to the other target attacking creature. It doesn't refer to Trygon Prime, even if you didn't choose a target.