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 blessed potion of see invisible.
- 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
See invisible is especially important in Slash'em. First, you might skip the invisible blessed rustproof Grayswandir on lvl 1 because you didn't see it. Also, pixies are the annoying equivalent of nymph, because they are always invisible. Therefore they can strip you naked, without you having a chance.