Simplify the following text for a Grade 12 native English-speaking person.
Original: HammerFall began when guitarist Oscar Dronjak quit Ceremonial Oath and invited Jesper StrÃ mblad ( from In Flames , also formerly a member of Ceremonial Oath ) to join him as a drummer on a new musical project which he had been idealising for some time .
Simplified: HammerFall began when guitarist Oscar Dronjak quit Ceremonial Oath and invited Jesper StrÃ mblad from In Flames to join him as a drummer on a new band he had been thinking about for a long time .