Ilk. historical development grammar.
The Ilkorindi were among the Elves who went on the great journey but chose to remain behind in Beleriand when the rest of their kin left for Valinor (LR/112). After this, the Ilkorin languages diverged from those of the other Eldar. As most of the Ilkorins were members of the third tribe, their languages were originally related to the (Middle) Telerin language spoken by the Sea Elves (LR/175). After the Noldorin exile to Beleriand, the Noldorin and Ilkorin languages strongly influenced one another (LR/177), but judging from the Tree of Tongues (LR/170), Ilkorin had more influence on Noldorin than the other way around. Regardless, many of the later phonetic changes of Noldorin resemble those of Ilkorin. After the fall of Beleriand, it seems that most of the Ilkorindi joined with the Noldor returning to the West, but a few remained behind, slowly fading away under the doom of Mandos (LR/178).
Reference ✧ LR/174 ✧ general