Defilers will start out with two heal-over-time spells, one ("Fertile Slime") being an instant cast that gives a light heal every 10 seconds with a 1 minute duration, and a stronger one ("Fungal Bloom") with a 2-second cast time, 5 second tick and 30 second duration. Spending some destiny points can earn them a targeted revival spell to resurrect allies, and at ranks 3 and 5 some stronger heals become available.
The other major facet of the Defiler will be their Curses, used to debuff the Freeps. One Curse is available for a new Defiler, "Curse of the Sticky Feet", which as you might imagine slows run-speed -- only by 10%, mind you, but there is apparently a trait that can improve this. Other debuffs become available at ranks 1, 2, and 7, and mainly work to lower the target's stats.
Dealing damage will primarily consist of throwing gourds full of nasty stuff at foes, with the abilities "Fire Gourd" and "Plague Gourd". Of course, that mean-looking staff isn't just for ritual chants -- you can also use it to "Head-crack" enemies into submission. Put all of these healing, debuffing and damage abilities together, and you have one formidable new ally for the Creeps. Check out the full developer diary to get more information about the Defiler's spells, and don't forget to read the prelude at the top!