Thingol SlainThingol slain (502 Y.S.) - Not by Morgoth or any of his minions.  When Húrin brought the Nauglamir he recovered from Nargothrond, and gave it to Thingol, the Sindarin King thought to mount the Silmaril that he and Beren recovered from Carcharoth into the golden necklace.  But he needed the Dwarves' skill to do it.


"Long was their labour; and Thingol went down alone to their deep smithies, and sat ever among them as they worked.  In time his desire was achieved, and the greatest of the works of Elves and Dwarves were brought together and made one; and its beauty was very great, for now the countless jewels of the Nauglamir did reflect and cast abroad in marvellous the light of the Silmaril amidmost." ~The Silmarillion, chap. 22 (Of the Ruin of Doriath)


But immediately there arose a dispute over whom the newly made treasure belonged to.


"Then Thingol, being alone among them, made to take it up and clasp it about his neck; but the Dwarves in that moment withheld it from him, and demanded that he yield it to them, saying: 'By what right does the Elvenking lay claim to the Nauglamir, that was made by our fathers for Finrod Felagund who is dead?'" ~The Silmarillion, chap. 22 (Of the Ruin of Doriath)


Thingol did not relent, but scorned the Dwarves.


"'How do ye of uncouth race dare to demand aught of me, Elu Thingol, Lord of Beleriand, whose life began by the waters of Cuiviénen years uncounted ere the fathers of the stunted people awoke?'" ~The Silmarillion, chap. 22 (Of the Ruin of Doriath)


That's when all hell broke loose.


"Then the lust of the Dwarves was kindled to rage by the words of the King; and they rose up about him, and laid hands on him and slew him as he stood.  So died in the deep places of Menegroth Elwë Singollo, King of Doriath..." ~The Silmarillion, chap. 22 (Of the Ruin of Doriath)


From that moment the Elves and Dwarves were at war (see Alliance Broken below)