User blog comment:Tails365/A true tank's role/@comment-6301771-20140122163434

Damage does not prevent someone from being a tank. And for that matter, Sejuani, Maokai, Zac, Rammus, etc. are all not only capable of dealing impressive damage, but should be building sources of damage that benefit their team.

The role of a tank is twofold:

1. Draw aggro to keep your squishy teammates from being murdered.

2. Disrupt enemies through CC.

If you're dishing out lots of constant AoE DPS (Sejuani, Amumu, anyone with Sunfire, etc.) you are going to be drawing aggro. That's what a tank is supposed to do. That's why Sunfire Cape is an item, and why items like Abyssal Scepter are frequently bought on tanks even if they aren't just a pile of health to staple onto yourself and be useless with. It's because this is a team game, and the goal is to kill the enemy team before they kill you. If you're the tank, your job is to ensure that happens however you can.