The bat I believe is the bad guy flying off, setting the possibility of a Fairlight 3 (which of course never happened). The bat first appears in the preceding screen:
The end sequence of the game starts when you get to the stationary monks:
The first monk was causing trouble as far back as 2000. It's impervious to spiky balls from the front, and the corridor isn't wide enough to launch one down its side then change its direction. The only way I've found to do this is to use the flying carpet to stand on the doorframe, take a single pixel step down onto the monk's head, and drop an unactivated spiky ball.
After that it's plain sailing till the penultimate screen (assuming you still have a working flying carpet and a full set of spiky balls with you, if you'll pardon the expression.)
This final screen has a locked exit door — but I don't know how you get through it (apart from with POKEs). The lock doesn't seem to match any of the keys, from either part of the game. Nor does it match the flying carpet, a spiky ball, the potion from part one (at least not the easily accessible one), the magic potion from part one which turns you into DragIsvar, the sword which frees the captain of the ship, or the magic wand in part one (which makes bubble damage not kill you). The only other possiblities I can think of are: the harder-to-get potion from part 1, the potion from part 2, or some kind of object-combination which you get after solving some task or other.
All these screenshots were taken from the 128K version — apparently some versions do not contain the ending sequence.