Kinslaying at Alqualondë
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When Fëanor decided to leave Valinor he needed ships to travel to Middle-earth, but the Noldor possessed no ships, and Fëanor feared that any delay in their departure would cause the Noldor to reconsider. The Noldor, led by Fëanor and his sons, tried to persuade the Teleri of Alqualondë, to give him their ships. However, the Teleri would not help them, since it was against the will of the Valar, and in fact attempted to persuade their friends to reconsider their decision to leave. Displeased with the Teleri's answer, the Noldor started taking the ships by force. This angered the Teleri, and they threatened the Noldor with rocks and arrows, and they threw many of Fëanor's Ñoldor out of the ships into the harbor. They also attempted to block the harbour.
Then the Noldor drew swords, and the Teleri their bows, and there was a bitter fight that seemed evenly matched, until the second Host of the Ñoldor, led by Fingon, arrived together with some of Fingolfin's people. Misunderstanding the situation, they assumed the Teleri had attacked the Ñoldor under orders of the Valar, and they joined the fight. In the end, many Teleri were slain and the ships were taken. The ones that continued towards Middle-earth were therefore cursed by Mandos.
 Later Kinslayings
The Sack of Doriath where Dior Eluchíl was killed by the Sons of Fëanor is referred to as the Second Kinslaying in The Silmarillion, and the attack by the Sons of Fëanor on the Mouths of Sirion where Elwing was attacked is called the Third Kinslaying. It was stated by Eönwë herald of Manwë that because of these evil deeds the remaining Sons of Fëanor had lost all right to the Silmarils.