The Kinslaying at Alqualondë:

The first evil of the Oath of Fëanor, and a major cause of the eventual Doom of the Noldor.
When the Teleri wouldn't join Fëanor or give him their ships, Fëanor led the host of the Noldor to attack the Teleri and steal the ships. Many of the Teleri were slain and as a result wouldn't join the Vanyar in the Great battle, though they were persuaded to sail the ships to Middle-Earth, not one of the Teleri set foot on land.

The Kin-Strife:

The civil war in Gondor between the years 1432 and 1448 of the Third Age, it was fought over favours shown to the Northmen and the fear that it was weakening the Dúnedain blood and shortening their lifespan. The King had to flee Gondor, but ten years later returned with an army and retook his country. The surviving rebels fled to Umbar, where they caused trouble until the time of the War of the Ring.

The Lord of the Rings
The Silmarillion
The Unfinished Tales
The Hobbit
The Complete Guide to Middle Earth by Robert Foster