Pretty sure it wasn't Yurgir.
Dwarf dude gives you some Infernal Alloy he found. Suggesting what came through Grymforge was one of Zariel's infernal machines. Yurgir is not associated with Zariel (He mentions he can smell the stench of her infernal machines if you bring Karlach along)
The meregon masks are everywhere. It even talks about how the meregon's need a commander to fight, questioning where the commander is. Which happens to be Yurgir.
Writing can be found in Grymforge talking about the infernal force that attacked. Describing Yurgir and the meregons.
Not to mention the mason in Reithwin is the reason they attacked ketheric's forces, due to the deal made. This can be found in the books in Reithwin as well as seen with the mason in the House of Hope.
It's also evident in the task that Yurgir has in the Gauntlet of Shar.
Yurgir and his meregons were 100% responsible for what happened in Grymforge.