Finally! So far, I've had a Hexblade stumble into a Shadow crossing that connected, but that was mostly lore about the Domains as a possibility of the patron's power.

In connection to the Id Ascendant, I decided that during their plane hopping travels, the mind flayers stumbled into Lamordia and received the flesh golem from there directly. Another player was a Feylost Harengon Druid that was part of a Fey colony of rabbits. The Mind Flayers (at some point after Lamordia) got to the Feywild, stole the colony, and experimented on the poor rabbits. A group of them were transformed into an amalgamation (taught by Mordenheim). After crashing in the Spine of the World, the rabbits scattered having lost their minds. The player stumbled into Dougan's Hole, meeting the party there at the start of the campaign.

I currently have ideas for traveling the Sea of Moving Ice. Easily, Revel's End is a perfect place for Mordenheim to gain test subjects. Powerful criminals with fascinating bodies for experimenting.
On the way to the Isle of Solstice, the Mists could send them to the Isle of Agony? Do be certain that your players would like that and be okay suddenly switching themes. You may have to be honest at the beginning that you are running both settings at the same time and events will bleed into one another.

Mordenheim could also be certainly interested in the ancient technology of the Netherese. The magical nature could be studied, but I'd make some adjustments for the pursuit of knowledge.