Diablo, the Prime Evil

Diablo, the Prime Evil gains +1/+1 for each demon Lord in your graveyard.
other Demons you control gain +2/+0 as long as Diablo, the Prime Evil is on the battlefield.
Other demon Lords cannot be on the battlefield at the same time as Diablo, the Prime Evil.
"The seven Evils are one within me! I am Legion!"-Diablo