using the seal of ascencion while taking part in the UBRS arena-event against rend blackhand causes an extra-event to start.

event id: 4622(?)
seal of ascension (must be equiped to use): item=12344
spell to start the event/summon the dragon in human form: spell=16349
transform into dragon-form: spell=16421
dragon-form: npc=10538
heal for entire raid: spell=16319
heal only for seal of ascension-user: spell=16332
resistance-buff: spell=16372
fire-resi-buff: spell=16373
frost-resi-buff: spell=16392
nature-resi-buff: spell=16391

the activation-spell of the extraevent shares a raid-wide cooldown fo 3 minutes, yet during the event it can only be used 1 time, while the whole arena-event is in progress)

what happens exactly depends on which part of the event is in progress (beginning/during the waves, figth against gyth, fight against rend)

• before/during the waves:
  -vaelastrasz spawns in human form
  -before each wave spawns, vaelastrasz heals each member of the raid by 5000 HP
  -before gyth spawns, he heal each member of the raid by 5000 HP
  -when rend falls down, nefarian banishes the human form vaelestrasz, he cant get into fight.
  -human form collapses after rend dies
• right before gyth spawns or at the beginning of the fight with gyth (until gyth has 50% HP(?))
  -vaelestrazs spawns in human form.
  -casts resistance buff upon the whole raid, which resistance is then appplied (fire, frost, nature) is random
  -heals the entire raid by 5000HP
  -when rend falls down, nefarian banishes the human form vaelestrasz, he cant get into fight.
  -human form collapses after rend dies.
• right before rend falls down (when gyth has under 50% HP)
  -vaelastrasz spawns in human form
  -heals the user of the seal of ascension by 91k (actual official number by blizzard, it can be "full HP", too) and restores 5000 mana (user of seal only)
  -when rend falls down, he transforms into dragonform and attacks rend

after rend is dead, more motes are screamed, the dragon itself collapses (dies and disappears)

emotes: (make sure to sync it with nefarians emotes during the event)

• upon spawning before/during the waves OR when he heals for 91k OR when he casts the resi-buff:
  -Vaelastrasz the Red yells: All is not lost! Battle these beasts without fear. Your wounds shall heal, bones I shall mend. Be renewed, heroes!
• when nefarian banishes vaelastrasz:
  -Vaelastrasz yells: You will suffer in this defeat, Neferain!
  -Lord Victor Nefarius yells: Enough! Playtime is over!
• when transforming into dragon:
  -Vaelastrasz the Red yells: You have come too far to fail. Stand back, mortals.
• while the dragon-form npc collapses OR when the human form collapses
  -Lord Victor Nefarius yells: Vaelastrasz, when this world belongs to the black flight, your flight shall know its end.
  -Lord Victor Nefarius yells: But for now, your death will have to suffice...
  -Vaelastrasz yells: You merely destroy an image, fool. I shall hunt you until the end of days.

proof:
added in patch 1.11.1, was never removed (until WoD, where UBRS became a hero)
http://www.wowwiki.com/Rend_Blackhand
https://www.youtube.com/watch?v=dL_LkwkI83Q (video of the event on a private server)
http://www.wow-pro.com/dungeon_and_raid_guides/60_jame039s_upper_blackrock_spire_guide_ubrs
http://wow.ingame.de/wiki/Vaelastrasz
http://outridersguild.proboards.com/thread/586
http://www.wowwiki.com/Seal_of_Ascension#Summoning_Vaelastrasz_During_Battle_with_Rend_Blackhand