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.
Best Answer
There are none which match your requirements, but you can customize an existing creature to give it this ability
There are no creatures in the D&D 5e Monster Manual which can shift between planes other than Ethereal without the use of an ability considered as innate spellcasting.
Some creatures around CR6 can use innate spellcasting or psionics to planeshift. These include the cambion (CR5, p.36), githyanki knight (CR8, p.160), githzerai zerth (CR6, p.161), night hag (CR5, p.178), and mind flayer (CR7, p.222).
As a DM, you can always simply invent your own monsters or modify an existing one so that it can plane shift. The easiest solution is to simply take an existing creature and make a variant that can shift between planes. This gives you an official and well-balanced monster which fills the niche required.
Making a variant monster in this way is an officially endorsed rule in the Dungeon Master's Guide, p. 273: