In an upcoming 5e campaign, my DM has approved me for a bit of an...odd combination.

I will be playing a Beastmaster Ranger Half-Drow, who, obviously not getting on well in society in the Underdark, took to messing about with the various beasts that wandered the area instead of other drow, before eventually making their way to the surface in search of a better life.

I have hatched and am raising a darkmantle, found before leaving home. It will just be a companion that doesn't do much and requires training to keep it manageable until I reach level 3 in order to use it as a Beast Master, but the issue is figuring out how to level it up as we go along.

I think some adaptation from the UA revised ranger would work, but I'm not sure how to calculate it using the base stats for a Darkmantle, as it obviously isn't in the approved list of companions. I'm also afraid of making it either over or under powered, since it's just outside the CR it "should" be, and has odd attributes.

Any suggestions on how to handle this would be great!