Soul Leech doesn't make much sense. The first tic would do about 200 damage to yourself to only block maybe around 20-50 damage depending on the attack. The next tic would be 190 damage for the same amount. As a tank you'll want to be high on HP in case of a big bad beastie bam. This means you'll almost always be taking 200 damage for mitigating much less.

Dark Soul suffers from the same problem. It takes around 1200 HP to mitigate around 200 HP in auto-attacks and maybe around 750 in special abilities. So you lose 1200 for around 1000 HP mitigation. The value is lost even further on blocks and parries (roughly 140HP in auto-attacks and 560 in special abilities for a total of 700 mitigation for 1200 cost if you parry/block them all). This is averaging around 75 HP for auto-attack damage and 1k for special ability damage and blocking/parrying at 25% reduction.

Unholy Ground looks exactly like Hallowed Ground when it comes down to actual functionality. Dropping your HP to 1 would put a TON of strain on the healers to get you back up to full. Since you reduce their effectiveness on you for the first 5s (I realize you did this for enmity issues with the DRK's pacification) that means they have 10s to get your HP up while maintaining low enmity, dodging attacks, and maintaining the HP of the rest of the group. It's not a very group friendly ability and that's what tanking is primarily about: How will this ability benefit your group?

The reason why stoneskin isn't utilized mid fight is because A: 3 second cast time and B: MP requirement. The issue we run into here with Drain is 3s of doing nothing. 3s raises the possibility of getting your cast interrupted by a large margin. All this for restoring some HP that might be already lost in the time it takes to cast. Actually, if it goes on their regular attack stats, using PLD as a guage it would only restore around 150 HP.

I'm not really sure on the purpose for Absorb. Would be nice in speed runs I suppose but otherwise there shouldn't be much issue with MP. The only ability I see here that uses MP is Unholy Ground.

I'm not sure a DRK could be a tank very easily. Using HP as a resource works better if you aren't the one being hit. You'd have to up the damage mitigation a lot to make it viable but then it could have some real balance issues. Like I said in your MYS thread, reusing animations would be pretty boring and again this looks like it will play the same as a PLD (FB>SB>RoH). I'm also one that prefers 2 roles stemming from every class.