Invisibility

Invisibility was a spell in demo version of Diablo I. In the released game, it was cut.

General Information
Upon casting, the spell would make the character invisible, making monsters ignore them for a period of time, useful to pass the dangerous area or escape from combat. Upon attacking or casting any other spell, the effect would fade. Apparently, it would not be able to fool some foes, most importantly bosses.

The spell seemed useful in early versions, when stealth and tactics were part of the plan, an alternative to massive brute force destruction.

The icon was later used for in Diablo II and its expansion.

Smoke Screen in Diablo III functions similarly to Invisibility.