Originally Posted by
Inactivity
Not true, it basically makes you and your pet share a health pool (similar to Valiona and Theralion). Damage taken is then split between the two of you so that you and your pet lose the same % of current health. Your health pools are not "readjusted" so for instance if you toggle Soul Link on and off, it won't magically give you and your pet the same % of health. Madness of Deathwing Heroic was bugged in that pets did not have the 90% AoE damage taken reduction against the parasitic backlash. So on fights with unavoidable AoE damage, you increase your effective health with this talent, since your pet only takes 10% AoE (110% damage taken) but your effective health goes up 25%.
If you use Grimoire of Sacrifice, Soul Link is a flat 20% health boost.
Sacrifical Pact is on a 1 minute cooldown and gives you a shield equal to about 50% of your health. If you use Sacrifice, then you sacrifice 50% of your health to get a shield equal to 100% of your health - this is based on current health so it assumes you are topped (in PvE mostly your pet will be topped if you don't use Sacrifice). Note that if you are using a pet, then you can use this ability even if stunned (if your pet is not).
Dark Bargain absorbs all damage for 10 seconds, then you take 50% of it (so this is a 50% cooldown) as a DoT over 20 seconds - on a 3 minute CD. You can easily soak massive attacks with this ability. The DoT is shadow damage and can further be reduced by unending resolve and twilight ward. As an illustration of its power, soaking an entire unmitigated, un-nerfed 25 heroic impale (1.2 million) leaves you with just a 30k/sec DoT for 20 seconds, which can easily be healed through with a few direct heals and some HoTs.
Basically, for generic damage I would take Soul Link if using Sacrifice (health boost) and Sacrificial Pact if using a pet. If there's massive, predictable bursts of damage I would take Dark Bargain.