How can i activate sleem from the box of rage???!! Please help!!!

[LalooHunter]

So, my problem is that I couldn't figure out how to activate my dear Sleem in the box....

I see some description when I click on his portrait like this:

"SATIETY:0/100%
Sleem will gobble up poisonous Swamp and Royal snakes and posionous Spiders from your army. The stronger the creatures eaten, the more satiatedis the swamp spirit"

So, how I understand it, this means that he will eat up some of my own creatures. I have now some poisonous units in my army but somehow, nothing has happened.

Maybe I have to give those units to him or are there any other ways to make Sleem serve me or it will just happen after a time that he will join me??!?

If someone knows the answer I would gladly appreciate his/her advice!!

PLEASE HELP ME, I'M CURIOUS !!!