Polukranos, Unchained

If the target creature is an illegal target when Polukranos's activated ability tries to resolve, the ability doesn't resolve. If Polukranos is no longer on the battlefield, the target creature won't deal or be dealt damage.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as an escape cost), add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was and no matter whether an alternative cost was paid.

Once you begin casting a spell with escape, it immediately moves to the stack. Players can't take any other actions until you're done casting the spell.

If an amount of damage would be dealt to Polukranos greater than the number of +1/+1 counters on it, all of that damage is prevented, and all of those counters are removed. In most cases, this will result in Polukranos having 0 toughness, so it will be put into its owner's graveyard.

If damage that can't be prevented is dealt to Polukranos, the damage is dealt and you'll simultaneously remove that many +1/+1 counters from it, all before the game checks for lethal damage.

Escape's permission doesn't change when you may cast the spell from your graveyard.