Rank 1 Gives your Soul Fire spell a 13% chance to Stun the target for 3 sec, and your Rain of Fire and Hellfire spells a 13% chance over their duration to Stun targets they damage for 3 sec.
Rank 2 Gives your Soul Fire spell a 26% chance to Stun the target for 3 sec, and your Rain of Fire and Hellfire spells a 26% chance over their duration to Stun targets they damage for 3 sec.