Midnight is a boss that will pull other creatures into the room if you do not first clear them out. At level 90, this is not really dangerous but can make the fight annoying.
IMPORTANT: Damage Midnight (and Attumen) very carefully. The fight has two phases and if you kill Midnight too fast the whole encounter has a good chance of bugging out - i.e. Attumen never mounts Midnight and simply fades away. If the encounter bugs out, it will not reset and you lose your opportunity at the mount until the raid lockout resets.