Bogmangler

Ambusher. In the first round of combat, the Bogmangler has advantage on attack rolls against any creature it has surprised. 

Multiattack. The Bogmangler makes two attacks with its vine whip.

Vine Whip. Melee Weapon Attack: +4 to hit, reach 15 ft., one target. Hit: 7 (2d4 + 2) bludgeoning damage.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) piercing damage plus 4 (1d8) poison damage. The target must succeed a DC 13 Constitution saving throw or be poisoned for 1 minute. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. 

Pounce. If the Bogmangler is underwater, it pounces on its enemy using its network of vines to propel it. It moves 30 ft in a straight line and makes a bite attack against a target in range. 

Entangling Plants (Recharge 5-6). Grasping roots and vines sprout in a 30 ft radius centered on the Bogmangler, withering away after 1 minute. For the duration, that area is difficult terrain for other creatures than Bogmanglers and plant creatures. In addition, each creature of the bogmangler's choice in that area when the plants appear must succeed on a DC 12 Strength saving throw or become restrained. A creature can use its action to make a DC 12 Strength save, freeing itself or another entangled creature within reach on a success.