Since the opponent controls a creature (the Recluse), Polukranos must target it with the ability. Because of deathtouch, Polukranos will die along with the Recluse, and you just spent a lot of mana to trade your Polukranos for a spider.

Does Deathtouch kill anything?

Deathtouch is an ability keyword in Magic: The Gathering. Whenever a creature with deathtouch deals any amount of damage to another creature it is enough to destroy it, regardless of the other creature's toughness. ... Giant Scorpion MTG card with deathtouch.

Is Polukranos Unchained good?

Polukranos, Unchained has promise; it's cheap and powerful, though it's extremely vulnerable to both burn and chip damage overall. Still, being able to come back after being removed is a great feature for a threat, and it's definitely a massive bomb in Limited!