Does this Zombie Swarm Homebrew require a grapple check?

Question

This homebrew Zombie Swarm found on dndbeyond.com has the following action:

Grab. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 14 (4d6) bludgeoning damage, and the target is grappled (escape DC 15). Until this grapple ends, the creature is restrained, and the swarm can constrict another target. If the hit points of the swarm is reduced by half, then reduce the damage by half (2d6).

Does this mean that the target is immediately grappled, and only gets a chance to escape the grapple on their turn, or that the "Grapple" special melee attack is attempted and both creatures roll their grapple check?

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I'm aware homebrew falls into a potentially wishy washy category but I'm not sure if this is something that other monsters have and therefore have rules we can apply.

Answer 1

It does not require a grapple check on the part of the attacker.

Assuming that these mechanics work the same as other monsters in the MM, the target is automatically grappled if the attack hits.

This ability is similar to those already in the MM. For example, the Kraken (MM 197) has this attack:

Tentacle. Melee Weapon Attack: +18 to hit, reach 30ft., one target. Hit: 20 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. The kraken has ten tentacles, each of which can grapple one target.

Other creatures that have attacks like this include the Marilith, the Otyugh, the Rug of Smothering, and the Grell (as well as many others).

If a creature gets hit by an auto-grappling attack like this, they have to spend their action on their turn in order to escape (PHB 195):

Escaping a Grapple. A grappled creature can use its action to escape...

As you can see, auto-grapple attacks are very powerful, since they can reduce your speed to zero and force the target to waste an action. The swarm seems to get as many such attacks as it wants, which seems awfully strong for a CR 4 enemy...