Bloodthistle Petal

Bloodthistle Petal is a caster buffing drug, grants +10 to spell power, served at Lord Saltheril's party for those who have finished the quest The Party Never Ends and are carrying in their pack. It is only usable by blood elves. It is essentially the same as, except it is free, BoP, and has a carrying limit of 5. Be aware that after the buff runs out, the character gets a Bloodthistle Withdrawal debuff, which reduces Spirit by 5 for 20 minutes, unless more bloodthistle is consumed (either kind), renewing the buff and delaying the withdrawal.

Source
This item is served for free on platters on the tables at the party in Saltheril's Haven in Eversong Woods. After you loot one, it will respawn after a short time.