Multiattack. The myrmidon makes three Flail attacks.
Flail. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit:8 (1d8 + 4) force damage.
Lightning Strike (Recharge). The myrmidon makes one Flail attack. On a hit, the target takes an extra 18 (4d8) lightning damage, and the target must succeed on a DC 13 Constitution saving throw or be stunned until the end of the myrmidon's next turn.
Elemental myrmidons are Elementals conjured and bound by magic into ritually created suits of plate armor. In this form, they possess no recollection of their former existence as free Elementals. They exist only to follow the commands of their creators.