Invincible is a flying mount obtained when defeating the Lich King "in his strongest form"on heroic difficulty in 25-player mode only. It can also be listed on the Black Market Auction House.
Here you can buy Invincibles Reins for EU or US Servers.
You'll get guide for getting Invincibles Reins
You'll be able to get Invincibles Reins
Additional budget for Character transfer or Character boost to max level. BMAH Services in Shadowlands now requirement to have a max level character on server, so Trial character will not work.
Free time on evening, if you choose selfplay option