Illidan could have broken the door, quite easily. It was flimsy, and he was not one do simply let things such as doors bar his way. However, it wasn't the door that gave him pause, it was the magic behind the door that did.

Interesting. He sniffed the air, but it wasn't the scent of the air, tinged with energy that caused him to pause. It was demonic. A kindred soul, perhaps, although he wasn't certain. It was weak, but if there were demons abound, it boded well for him. He turned, and waited for the creature, his arms crossed, and an imperious look on his face. He waited to see what it would be. He doubted it would be anything like the demons he had known or faced back on either Azeroth or Draenor.