I used to use it in the abyss, but now I don't bother.
Best possible use: you can use it to break into that end vault in the crypt that looks like a skull,
without alerting the caged liches to your presence.
- Code:
##############################################################################
# eye to eye with the devil
#
NAME: david_glass_crypt
ORIENT: float
TAGS: no_item_gen no_rotate
PLACE: Crypt:5
MONS: lich / ancient lich
MONS: skeletal dragon / curse skull / curse toe
MONS: flying skull / skeletal warrior / w:40 nothing
KFEAT: ^ = alarm trap
MAP
.....................
.....................
..vvvvnnnnnnnnnvvvv..
..vlllllllllllllllv..
..vlllllllllllllllv..
..vllll..|||..llllv..
..vll...|||||...llv..
..vl.....|||.....lv..
..vl......@......lv..
..vmm..F.vvv.F..mmv..
..v1m...^^^^^...m1v..
..vvvvvvv+++vvvvvvv..
...vv..2.....2..vv...
..vv.333333333.vv..
...vvv.......vvv...
...vv+++++vv...
.............
Step 1: equip the lantern of shadows. This allows you to stand on the space I've marked @ without the liches seeing you.
Step 2: Get yourself flying somehow. The floor has pressure plates that drop the walls if you walk on them, and you can't use lev because you want the loot.
Step 3: Control teleport to the @. This is the riskiest step. Teleports have an error of +/- 1. This could put you in the adjoining squares- where at least one lich can see you. If you're lucky (and stealthy) it won't notice you in the one turn it takes to walk to the blind spot. You could also land in the wall you're trying to hide behind (the vvv) which means you get randomly ported anywhere- possibly right in front of the liches. Odds are though you end up outside the vault, and try again.
If you
do alter the liches, bail, wait for them to calm down, then restart.
Step 4: apport loot. (a mele character could do everything up to this step with scrolls / evocations. But c'mon, you can manage a level 1 spell).
Step 5: loot loot.
Step6: Leave via control tele.
This method isn't completely foolproof- for instance there are other crypt ends now, so it doesn't show up as often. Also, monsters can be spawned inside, and you'll have to deal with them first (possibly by opening the front door, then luring them away while running from the summon spam. Then come back when it's quiet to 'port in).