The N'Garai Cairn is a fictional item in the Marvel universe which first appeared in 1975 in issue #96 of the 1st volume of Uncanny X-Men.
The cairns are used by the demonic N'Garai to cross over into other dimensions. Wherever the walls between dimensions are thin, one of these can appear, thrusting upward from the ground. The number of N'Garai that can come through one of them varies and multiple cairns can simultaneously appear on a world, the start of a N'Garai invasion. One can also travel to the N'Garai dimension via one of these cairns.