Immutable Form. The golem is immune to any spell or effect that would alter its form.

Magic Resistance. The golem has advantage on saving throws against spells and other magical effects.

Magic Weapons. The golem's weapon attacks are magical.

Multiattack. The golem makes eight slam attacks. individually

Slam. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage.

Slow (Recharge 5–6). The golem targets one or more creatures it can see within 10 feet of it. Each target must make a DC 17 Wisdom saving throw against this magic. On a failed save, a target can't use reactions, its speed is halved, and it can't make more than one attack on its turn. In addition, the target can take either an action or a bonus action on its turn, not both. These effects last for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Zakum can cast Counterspell Unlimited times but only once during a round

Zakum has 3 legendary actions per round

1 Legendary action:
Slam Attack: 2 attack
Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage.

2 Legendary Actions:
Summon Undead: Zakum can summon up to 1d8 zombies or skeletons within 30 ft around itself

3 Legendary Actions: (can only use this once every 2 rounds)

Negative Energy Flood: You send ribbons of negative energy at one creature you can see within range. Unless the target is undead, it must make a Constitution saving throw, taking 5d12 necrotic damage on a failed save, or half as much damage on a successful one. A target killed by this damage rises up as a zombie at the start of your next turn. The zombie pursues whatever creature it can see that is closest to it.