I am preparing to DM a campaign in a world of my own design, and I require a monster to fill an important plot function.
The campaign is set in a pocket plane that is almost entirely cut off from the wider multiverse. As part of the plot, the characters are sent to hunt a monster that has managed to travel onto the plane due to its innate inter-planar abilities.
The PCs will be level 5 or 6 by this point.
I was originally looking at Phase Spiders, but having looked them up, their abilities are more about phasing between the material and ethereal planes, not so much travelling from one plane to another.
Are there any monsters in the 5e MM of up to roughly CR 6 (need not be exact), or from earlier editions that might be converted, that have natural inter-planar abilities?
It has to be a natural ability rather than a spell, since the idea is that the corpses of the creatures can be harvested for a material to be used in a planeshifting ritual for plot purposes.