I don't believe that Letum Frigus was where Mulcarn killed Sucellus, but it probably wasn't too far away. It is rather the place where Mulcarn entered Erebus the first time, and a small piece of his vault that remains in this world. Sucellus was killed close to where Taranis was held on the AoI map. Mulcarnw ould not have been able to defeat Sucellus had not the god of nature been distracted by his attempt to kill Taranis, whose unchanging nature made it impossible for him to be hurt (at least by the god of nature, whose attacks would by his nature cause gradual changes/injury rather than destroying all at once; Bhall probably could have killed Taranis with one hit) and allowed him to keep the god occupied even though he had no hope of actually winning.
The name Letum Frigus, meaning "Cold Ruin," was not applied to the place until Auric Ulvin found it (although it is anachronistically called that in AoI). During the Age of Ice, it was not a ruin at all, but a grand palace. I believe that when Mulcarn ascended he brought a part of his vault with him, specifically the peak Mount Mulyr (I'm not on my computer right now, and think I may have misspelled that) where his palace was located. After Mulcarn was slain, the palace began to fall apart, considering that it was made mostly of ice. The power of winter remained strong here, but it was enough to keep the palace completely frozen and so it left a collapsed mountain filled with empty caverns instead of grandly decorated halls of crystal.
Erebus is a flat, infinite plain, so it has no poles and can't really have a center either. All we really know about Letum Frigus's location is that it is near the lands where the Illians live, but that the Illians have to much fear and reverence for the spot to enter unless specifically told to by their god.
On the AoI map, it is west-southwest.