Everyone who is saying FFXI DRK was aggression only that was only during 75 cap content. DRK got a Total of 7 defensive skills after they raised the level cap to 99.
  • Drain, a self heal
  • Drain 2, a self heal that boosted max hp
  • Dark seal, increased the amount of hp you could drain through accuracy
  • Nether Void, increased the amount of hp you could drain
  • Dread Spikes, drained hp every time you took damage and gets a bonus from max hp increase
  • Catastrophe, weapon skill that drains hp
  • Entropy, weapon skill that drains mp.

So you could do combos like this.
Dark Seal > Drain to full hp > Nether Void > Drain 2 to max hp +20% > Dread spikes > weapons skill of your choice.

Stacking Nether Void onto Drain 2 then dread Spike with that hp bonus did a ton of mitigation, and If you needed more mp to cast drains and dread spikes you had Entropy to get mp back. Or if you took a few hits and still have enough mp you could top off your hp with Catastrophe.