First of all, Keeper of the 7 keys pt 1 isn't helloweens first album, they released thier first one in 1985, and have been around longer since. Based on the true definition of power metal, which is the melodic Iron Maiden sound mixed with high thrashy speeds, they were the first to do it pretty much. Iron Maiden aren't really "true" power metal, they were lacking the speed. Its like when people say Possessed were the first death metal band instead of Death, but both bands were around since 1983, and Posessed, while they released their first album in '85 and Death in '87, they were more of a thrash-death transition. Scream Bloody Gore was the first pure death metal album. Therefore, Death were the first Death metal band.