As stated above, the basic structure of the Immortal Cleric is Intervention 3 + Lots and Lots of Mana Potions. From there, it's player choice.
Because the Immortal Cleric build plans on repeatedly using Intervention, which will drain your mana poor, the following are decent design considerations.
1) Join Warriors Guild for Duel and Heavy Hand.
2) Don't worry about having too much mana or Health, so put a lot of points into STR.
3) Don't focus on skills that require a lot of mana, so passive or low-mana skills are preferred.
One of the more common versions on the Immortal Cleric is a build that's focused on combat (so lots of STR) with less concern about defense, mana pool, or health.
Any Cleric can add Intervention as a skill. It's useful as back-up for those tough fights or unexpected traps. The Immortal Build is designed to depend on using Intervention.