Greataxe. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit: 15 (2d12 + 2) [Slashing] damage. 

Call Undead. The giant roars and summons 1d4 zombies/skeletons to help it. They rise up from the ground or from nearby corpses with the leftover necrotic magic from its summoner.

Undead Fortitude. If damage reduces the giant to 0 hit points, it must make a constitution saving throw with a DC of 7 + damage taken, unless the damage is radiant or from a critical hit. On a success, the giant drops to 1 hit point instead.

Enter the description for how legendary actions work for your monster here.

Legendary Action Name (Costs # Actions). Enter the description for your legendary action.