See invisible is an intrinsic which enables you to spot invisible monsters and yourself, if you are invisible. It is very useful to possess as high level spellcasting monsters can render themselves invisible, and some monsters (notably stalkers) are intrinsically invisible.
Invisible monsters can also be seen with telepathy, barring the possibility of a mindless creature getting zapped with a wand of invisibility
The ability to see invisible can be acquired in the following ways:
- Drink a potion of see invisible (if non-blessed, will be temporary).
- Eat a stalker while invisible (note that this also gives permanent invisibility).
- Be crowned by your god.
- Sit on a throne (a small chance).
- Drink from a fountain (a small chance, and can also have bad effects).
- Wear a ring of see invisible.
- Be a Monk or a 15th-level Ranger.
- Polymorph into a monster with intrinsic see invisible.
One possible negative side-effect of only seeing invisible temporarily is that peaceful monsters might make themselves invisible (since you can then still see them), but once you can't see them anymore (e.g., by removing the ring of see invisible), they stay invisible. Running into them will then anger them, which might end in a quick death.
Monsters with intrinsic see invisible
- h mind flayer
- h master mind flayer
- y black light
- A Aleax
- A Angel
- A Ki-rin
- A Archon
- All dragons
- E Stalker
- Y Sasquatch
- All non-undead elves
- @ High priest
- @ Wizard of Yendor
- @ Croesus
- & Marilith
- & Ice devil
- & Pit fiend
- & Balrog
- All demon lords and demon princes
- The riders
- & Minion of Huhetotl
- D Chromatic Dragon
- D Ixoth
- @ Master Kaen
- & Nalzok
See invisible is especially important in SLASH'EM. Pixies are the (even more) annoying equivalent of nymphs, because they are always invisible. Therefore they can strip you naked, without you having a chance to fight back.
Invisible items are exceedingly rare, but importantly, even invisible non-artifact weapons are good, since they give +3 to-hit versus monsters which cannot see invisible. Finally, imagine the (exceedingly unlikely) annoyance if you skip a randomly generated Grayswandir on Dlvl 1 because you couldn't see it.