Byleth, Tested Professor

Whenever Byleth, Tested Professor enters the battlefield, put a +1/+1 counter for each other Legendary creature you control.
If a Legendary Creature were to die at anytime, you may remove a +1/+1 counter on Byleth, Tested Professor to give that creature indestructible. Tap this unit.
{5}{g}{r}{r} Exile Byleth, Tested Professor and return it to the battlefield under your control transformed.