At the beginning of your end step, bolster X, where X is the number of tapped creatures you control. (Choose a creature with the least toughness among creatures you control and put X +1/+1 counters on it.)

When Mjölnir enters, attach it to target legendary creature you control.

Whenever equipped creature attacks, tap target creature defending player controls and put a stun counter on it. Then Mjölnir deals damage to each opponent equal to the number of tapped creatures that opponent controls.

Equip

At the beginning of your end step, each opponent loses life equal to the number of tapped creatures you control.