The Altar will probably be found in the ruins of an underground temple. In my campaign the place was dedicated to Tharizdun, but it would work with most lovecraftian entities, and maybe for some demon lords as well.
The stats are given for D&D5, and it was designed to keep a small 4th level party occupied; feel free to scale the effects to fit your players. The idea is not to kill or seriously harm the PCs, rather to make them spend (or lose) some time in a kind-of-fun way.
The altar is a block of rough stone with a pink hue. Or maybe it is pure dark grey? It looks a bit greenish to me, too. It is kind of a rectangular box, but on a closer look its weird angles don’t make any geometrical sense, kind of something carved from a M.C. Escher drawing. A character trying to figure out exactly what it looks like, or having a close look at it, has to make a Wisdom saving throw (DC 13) or be stunned for 2d4 rounds.
Physically touching the Altar will trigger a random effect. If a PC uses a proxy effect like a Mage Hand spell instead of just touching the stone, give them a DC 13 saving throw to avoid triggering the effect; if they are using a spell, use their spellcasting ability saving throw, else Wisdom will probably do. In my campaign the party warlock prodded the Altar with a Mage Hand; it triggered the first effect on the list and got the Hand destroyed (probably wrong by the rules, but it felt right at the time).
To determine the actual effect, roll 1d8. All saving throws are DC 13.
- Arms of Hadar (as the spell) centered on the Altar.
- Character is blinded for 2d6 rounds, Constitution saving throw to avoid the effect entirely.
- Character is deafened for 2d6 rounds, Constitution saving throw to avoid the effect entirely.
- Caracter is confused (as in the Confusion spell) for 2d4 rounds, Wisdom saving throw to avoid.
- Darkness (as the spell) centered on the Altar, lasts for 2d6 minutes.
- Character is affected by spell Dissonant Whisper.
- The Altar casts Fire Bolt on the PC, at +5 to hit.
- Silence (as the spell) centered on the Altar, lasts for 2d6 minutes.