The Intercession skill is a class only skill that is gained from the Priest profession. With this skill a priest is able to intercede on behalf of the recently departed and aid them in reaching whatever plane of existence they might believe in after death.
As a reward for performing good deeds the priest receives a special temporary buff.
Skill Use
The Intercession skill is incorporated into the loot screen, after you kill a monster one of the available choices at the top of the screen will be to Intercede.
Intercession can only be used on monsters of level 300 or higher.
You should only intercede after taking any items or money from the corpse that you wish to keep as intercession will destroy the corpse and all possessions currently left on it.
Formula
Intercession = ((Target Level * 5) - 5)
i.e. To intercede on a level 300 monster you need 1495 intercession skill ((300 * 5) - 5) .
Successful use of the intercession skill will provide experience and a guaranteed purple skill gain.
Interceding will also activate one of several hundred different spell effects as a bonus reward for doing a good deed. The type of spell effect you activate depends on your targets level as well as the classification of the corpse.
These spells last around 16 hours, and only one such spell can be in effect at any one time although if you try to intercede while one spell is in effect it will either replace the spell with the new spell or increase the current spells duration so that it lasts 16 hours again.
You can view a full list of the spells available by going to Library > Spells and looking up the Intercession spell list.