Actions

Multiattack Smough makes two attacks with his Hammer.
Hammer Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 3d10+6 bludgeoning damage.
Knockdown Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 3d10+6 bludgeoning damage and the target must succeed on a DC 19 Strength Saving Throw or be knocked prone. 

Legendary Actions

Smough can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time and at the end of another creature's turn. Smough regains spent legendary actions at the start of his turn.

Execution : When Smough is hit by a melee attack, he can make a Knockdown attack against the attacker.
Butt Slam (Costs 2 Actions) : Smough slams his butt on the ground, causing all creatures within 5 ft. of him to succeed a DC 19 Dexterity Saving Throw, or take 4d12 bludgeoning damage and be knocked prone.
Fallen Comrade (Costs 3 Actions) : If Ornstein dies, Smough can crush him with his hammer to absorb his power. Smough gains 30 temp HP, his attacks deal an additional 6d8 lightning damage. In addition, he loses his Vulnerability to Lightning damage and instead gains Resistance to Lightning damage.