I was wondering what level and class is Demonknight to cast Power word kill (mage level 9 spell) and symbol: pain (druid level 7 spell). He should be a fighter-mage-cleric or something like that of a HIGH level, unless he is casting scrolls.

In IWD you have to be an 18th level Mage to cast lvl 9 spells and a 14th lvl cleric or druid to cast 7th level spells.