The quick answer
An Omen Idol is an Idol that drops already corrupted, in a 1 × 3 or 3 × 1 shape. It can carry class affixes normally reserved for larger Idols. It is distinct from an Idol Altar and from an ordinary Unique Idol.
Get and equip one
Omen Idols drop from Omen Window rewards. In 1.5, these events can appear from area level 18; Idol Altars can appear from area level 32. The old area-level-10 event threshold is outdated.
Check the item’s Omen type, shape, affixes and corruption marker, then fit it into a compatible space in your Idol Grid.
Why a second one may not fit
By default, you can equip one Omen Idol at a time. Some Idol Altars increase that limit. An Altar sits above the grid, changes its layout and can create purple Refracted slots that add effects to the Idols placed there.
Check the equipped Altar’s actual layout and Omen limit before treating a failed second placement as a bug. Both Altars and Omen Idols can drop from Omen Windows; no guaranteed drop rate is established here.