i go to UBRS a lot of time , trying to collect gear and complete quests for my paaldin and warrior , rend fight is kinda fun and if you don't have stupid people its nearly trivial

anyway one time i was there and suddenly i saw a dragon in the middle of the arena ( while we were fighting Rend ) i paniced and throw Divine Intervention on the paladin because i thought it was a wipe , but it wasn't

another time with my warrior some said i'll summon now while fighting rend and saw the dragon again

is there a quest for that thing or what ?

The key to UBRS (which is actually a ring, the Seal of Ascension) has a double use - not only does it unlock the door to UBRS, but it also helps you during the Rend event.

The ring is given to you by a friendly dragon, who's been spying on Nefarion and is trying to stop him from creating his chromatic brood. When he gives you the ring at the end of the quest, he says something to the rough effect of, "it will also help you im times of great need".

If you're wearing the ring during the Rend event, then whenever Gyth hits you with one of his random AE effects, the ring will counteract by giving you a resist buff. Additionally, you can use the ring to summon the projected spirit of the friendly dragon who helped you - he will spawn, cast a room-wide AE heal on your raid, then start meleeing Rend. Eventually, Nefarion gets ticked off and blows him up.