Fromost the Grimm
Explorer
Personally, I'd find any such 'letter' at the end of BBEG fight a bit of a let-down.
There are other ways of reaching the same thing, as other posters have chimed in with ... the crux of which comes down to how much your players play just to kill things, versus how much your players play for the exploration and investigation side of the game. If they just want hack n' slash, then they'll probably be happy with a simple one-degree-of-separation 'clue' on the BBEG. But if they like snooping about the dungeon, searching through the enemy commander's quarters, then that's the way to feed them the next clue. Moreso, planting a series of clues that point in a certain direction when taken in context of one another rewards that snooping.
Have orders in the BBEG's quarters written in a strange script, or bearing a seal or mark of a merchant house. A strange piece of jewellery or a unique weapon could yield information when it's taken to an expert to be appraised. Bizarre spell components could give a clue to another region of the world, and/or a unique holy symbol may lead to the party to another city looking for information.
And if the party don't put two n' two together with all this, send in an assassin! The party think they'd cleared out the dungeon and defeated the BBEG ... but they missed his sneaky assassin, who tailed them as they left the dungeon, and attacked one unsuspecting night. When the party finish him off, he's got another more obvious clue that can guide them in the right direction ...
There are other ways of reaching the same thing, as other posters have chimed in with ... the crux of which comes down to how much your players play just to kill things, versus how much your players play for the exploration and investigation side of the game. If they just want hack n' slash, then they'll probably be happy with a simple one-degree-of-separation 'clue' on the BBEG. But if they like snooping about the dungeon, searching through the enemy commander's quarters, then that's the way to feed them the next clue. Moreso, planting a series of clues that point in a certain direction when taken in context of one another rewards that snooping.
Have orders in the BBEG's quarters written in a strange script, or bearing a seal or mark of a merchant house. A strange piece of jewellery or a unique weapon could yield information when it's taken to an expert to be appraised. Bizarre spell components could give a clue to another region of the world, and/or a unique holy symbol may lead to the party to another city looking for information.
And if the party don't put two n' two together with all this, send in an assassin! The party think they'd cleared out the dungeon and defeated the BBEG ... but they missed his sneaky assassin, who tailed them as they left the dungeon, and attacked one unsuspecting night. When the party finish him off, he's got another more obvious clue that can guide them in the right direction ...