Kinslaying

Warning: Wikipedia contains spoilers

When Fëanor left Aman he needed ships to get to Middle-earth without great loss. So he tried to persuade the Teleri to give him their ships. But the Teleri would not help in any way against the will of the Valar. So the Noldor started taking the ships and sailing them away. But the Teleri saw the ships being stolen and threatened the Noldor with rocks and arrows and threw many of Fëanor's Noldor out of the ships into the harbor.

Then the Noldor drew swords, and the Teleri their bows, and there was a bitter fight that seemed even. But Fingon led some of Fingolfin's Noldor to the Haven and they saw many Noldor being slain, and they knew not that the Teleri were innocent. So it ended up that Fingon's Noldor bitterly slew many of the Teleri, and Fëanor's Noldor won the ships, only to be cursed by Mandos.