Hellraiser Goblin's ability also affects itself. If it enters the battlefield before combat, it will have to attack that combat if able.

If, during your declare attackers step, a creature you control is tapped or is affected by a spell or ability that says it can't attack, then it doesn't attack. If there's a cost associated with having a creature attack, you aren't forced to pay that cost, so it doesn't have to attack in that case either.

Unlike many other effects that force a creature to attack if able, Hellraiser Goblin's ability applies during each combat. If a turn has multiple combat phases, creatures you control must attack in each of them if able.