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?
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.