WoWDB

Ra'sha is a level 91 Rare in a cave guarded by two Zandalari Execution Guards. Ra'sha is one level lower than the other rares on the island, has two-thirds of the max health of the second lowest rare, and is not included in the meta achievement. Based on this, I believe he is some sort of "mini-rare", and he may drop  Key to the Palace of Lei Shen and  Shan'ze Ritual Stone at a lower rate or not at all. He sometimes drops a special "toy" item,  Ra'sha's Sacrificial Dagger.

The strategy for him and the strategy for the guards is the same; DPS him/them down as quickly as possible but watch your health and stay above 40-50% at all times since he/they can nearly insta-gib you towards the end with his/their  Bloody Strike's buff. Ra'sha has an additional pair of pairs that the guards do not, but neither is a massive threat. The first new spell is  Mark of Blood, and it is fairly important to watch for. If you ignore it, you will likely heal Ra'sha back to full health, and then you'll have to take him down again while he maintains his higher Bloody Strike stacks. The other new spell is  Bloodbolt, but it is fairly harmless. If you have a reliable interrupt, then by all means interrupt it towards the end of the cast so that Ra'sha wastes time, but otherwise don't worry about it.

This may or may not be fixed by the March 7th Hotfix "Ra'sha now scales their health based on the number of players they're engaged in combat with." depending on whether or not loot-sharing is part of health-scaling by default.

Edit: Actually I just killed him alone and had no dagger drop. I did get a  Small Bag of Zandalari Supplies though.