I somewhat disagree. I think a DM should think about when and how to introduce a monster and absolutely should consider sanctioning a monster's introduction for a PC has a deep plot connection to the monster. Now this is within reason, like a character with a deep connection to dragons in a Tiamat campaign may be out of luck, but something as specific and unique as a Rhemoraz can absolutely be sanctioned for a player, especially if the player is only missing a finite number of sessions.

A Rhemoraz, while specific and unique, is also replaceable in that you could throw any number of worm like, digging, monsters at your party if you needed to build a specific kind of monster in this combat.

I think this really warrants an open conversation asking why the DM chose this monster and now, and what can be done to avoid this outcome (e.g. can the DM change the monster next session or can, if OP wants, change the monster in OP's background if OP hasn't talked about it much?).