If a warlock's demon is Banished, this effect will be removed until the demon is back under control of the master.
During the MoP beta, it had the flavor text "The strongest of bonds is one which shares pain and pleasure equally." by Kanrethad. It becomes available to warlocks who complete the green fire quest chain.
Patch 7.0.3 (2016-07-19): Changed from a talent to a passive Demonology ability. Effective healing reduced to 2.5% for the warlock but increased to 5% for the demon (was 3%).
Hotfix (2013-09-13):Soul Link is no longer incorrectly being applied to the second demon summoned through Grimoire of Service.
Patch 5.4.0 (2013-09-10): Now transfers 20% of damage taken by the Warlock to their pet, and 3% of all damage dealt by the Warlock will be returned as healing for the Warlock and their pet. This ability is always active while the Warlock's pet is active.
Patch 5.0.4 (2012-08-28): Now a talent and redesigned. When active, all damage and healing you and your demon take is shared, but your demon's health is reduced by 50%. Recasting this spell cancels the effect.
Patch 4.1.0 (2011-04-26): Will no longer be removed by mounting when in a Rated Battleground.
Patch 4.0.1 (2010-10-12): No longer a talent. It is now a trainable spell.
Patch 3.3.0 (2009-12-08): Damage transferred via this ability can now be prevented by damage absorption effects.
Patch 3.0.3 (2008-11-04): Shared damage increased to 20%.
Patch 3.0.2 (2008-10-14): Bonus damage removed. Shared damage reduced to 15%, down from 20%. Moved to tier 3.
Patch 2.1.0 (2007-05-22): Adjusted tooltip to explain that the damage transferred to the pet cannot be prevented. The functionality remains unchanged. This ability will now work with enslaved demons.
Patch 2.0.3 (2007-01-09): The damage increase component has been improved. The damage mitigation component has been reduced.
Patch 1.12.0 (2006-08-22): Soul Link will persist across instance boundaries.
Patch 1.11.0 (2006-06-19): If your pet is banished, Soul Link will stop functioning until Banish wears off. In addition, Soul Link's tooltip has been clarified to indicate it only works with your imp, voidwalker, succubus, or felhunter.
Patch 1.9.0 (2006-01-03): Can no longer be dispelled off the warlock. In addition, Soul Link can no longer be used on non-demon pets.
Patch 1.8.0 (2005-10-10): Percentage of damage split with pet has been reduced. Is now a spell cast on your pet that gives an aura to both you and the pet. This aura increases all damage caused by you and your pet by 3% and splits 30% of all damage taken by the caster to the pet. The aura will last as long as the pet is active, and the pet must be the target of a dispel effect in order to dispel this ability.
Formerly redirected 50% of all damage and lasted for 30 seconds, but was dispellable.
Patch 1.7.0 (2005-09-13): Fixed a bug where Soul Link and Power Word: Shield, when used together, would heal the Warlock instead of splitting or absorbing damage.
Hotfix (2005-07-28): Hotfixed a bug which was preventing warlocks from targeting their pets with various spells and abilities, including Soul Link.