Grab. As a bonus action, and if only the crawler have advantage on his attack, the crawler can attempt to grapple an creature Medium or smaller. (Athletics vs. Acrobatics or Athletics)
Pack Tactics. The crawler has advantage on an grapple against a creature if at least one of the crawler's allies is within 5 feet of the creature and the ally isn't incapacitated.
Devour Prey. The crawler may only use its bite attack if it has advantage on the attack or if the target's speed is reduced to 0 by any means, such as being grappled, paralyzed, or incapacitated.
Fungal Fortitude. If damage reduces the crawler to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is necrotic or from a critical hit. On a success, the crawler drops to 1 hit point instead.
Parasite. If the crawler reduces a humanoid to 0 hit points with its bite attack, it becomes infected with fungal spores. It is unable to regain hit points unless an effect equivalent to the Lesser Restoration spell is applied to it. After one minutes, the creature transforms into an runner with 1 hit point if the infection is not cured.
Evasion. When the crawler is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it instead take no damage if it succeeds on the saving throw, and only half damage if it fails.
Multiattack. The crawler makes 2 slam attacks.
Slam. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage plus 11 (3d6) poison damage.
Bite. Melee Weapon Attack: +8 to hit, reach 5ft., one target. Hit: 22 (4d8+4) piercing damage plus 14 (4d8) necrotic damage. On a successful hit, the target must succeed on a DC 14 CON saving throw or become poisoned for 1 minute.
Tongue. The crawler can attempt to use his large tongue to pull a creature toward him. The targeted creature makes a 15 DEX saving throw. If it fail, its grappled and moves 20ft. towards the crawler. The creature can attempt to escape this grapple by cutting the tongue out, causing 15 damage to it, or make a 14 athletics or 14 acrobatics saving throw.