They were continuing John Dee's mission. Except sorta fictionalized and made EEEEVVVIIILLL.
As in, one great British Empire spanning the world, under fear. Just as demonic space demon angel god intended. It's also why the endboss is Talbot (Dee stand-in) as opposed to Marlowe (Elizabeth stand-in). With Drake obviously standing in for his adoptive ancestor. IT'S SYMBOLIC!
The Djinn jar was some sort of super hallucinogenic agent. Solomon (egggghhhh, poorly researched, perhaps?) sealed it in a jar then put it down there trying to contain the "magic" (hallucinations). Turns out, whatever's in there is water soluble. Whoops.