Legendary creatures don't untap during their controllers' untap steps.

When Arena of the Ancients enters, tap all legendary creatures.

Enchant creature you control

When Cocoon enters, tap enchanted creature and put three pupa counters on Cocoon.

Enchanted creature doesn't untap during your untap step if Cocoon has a pupa counter on it.

At the beginning of your upkeep, remove a pupa counter from Cocoon. If you can't, sacrifice it, put a +1/+1 counter on enchanted creature, and that creature gains flying.