This spell costs less to cast for each other spell cast this turn.
Trample, haste
Trample over planeswalkers (This creature can deal excess combat damage to the controller of the planeswalker it's attacking.)
Thrasta has hexproof as long as it entered this turn.
Cast a flurry of cheap spells to make Thrasta nearly free, then swing for 10 trampling haste damage the turn it lands while it's protected by hexproof. As a mono-green storm-adjacent beater, you chain rituals and cantrips to deploy Thrasta repeatedly and pump it to lethal commander-damage range in one or two attacks.
Hexproof on entry makes it nearly impossible to remove before combat, surviving spot removal
Haste plus trample means immediate, hard-to-chump damage every time it lands
Mono-green gives cheap, consistent ramp and recursion
Cost reduction can make a 12-mana 10/10 cost almost nothing in a spell-heavy turn
Hexproof only lasts the turn it enters, so it dies to sorcery-speed wipes and removal on later turns
Needs 21 commander damage or repeated big swings—one body is fragile to fogs and blockers
Mono-green lacks card draw and interaction to protect a developing board
Color identity locks you out of free counterspell-bait spells from other colors that fuel cost reduction
Legendary Creature — Dinosaur
less to cast for each other spell cast this turn.