Oh. By reading wiki i have got some understanding now.
The Islamic Holy War CB vassalises where possible.Oh. By reading wiki i have got some understanding now.
He is known demon worshiper. Which allow me to HW him even he has same religion.
UPD - also winning common Holy War against his capital duchy made him my vassal. Which is somehow weird.
Where possible. But here I vassalized whole kingdom of Denmark with one holy war CB.The Islamic Holy War CB vassalises where possible.
I didn't realise you could holy war demon worshippers though! Useful.