Stalwart Defense. The amalgamate can take 2 reactions on each round of combat.

Infectious. When the amalgamate kills a humanoid, the humanoid rises after 1 hour as a zombie that is friendly to the amalgamate.

Multiple Heads. The amalgamate has advantage on Wisdom (perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Multi Attack. The amalgamate uses its tendril and makes three attacks; one with its greatsword and two with its shortswords .

Shortsword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) slashing damage.

Greatsword. Melee Weapon Attack: +6 to hit, reach 5 ft., two targets. Hit. 9 (2d6 +3) slashing damage.

Tendril. The amalgamate reaches out its tendril at one creature it can see within 10 feet. The target must make a DC 16 Strength saving throw or become grappled. The amalgamate can only have one grappled at a time. If the amalgamate takes this action while it has a creature grappled in this way, the creature takes 8 (2d8) bludgeoning damage and is thrown 10 feet to an unoccupied space the amalgamate can see.

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

Sentinel. When a creature moves within 5 feet of the amalgamate, it can make one shortsword attack against that creature.

Parry. The amalgamate adds 2 to its AC against one melee attack that would hit it. To do so, the amalgamate must see the attacker and be wielding a melee weapon.