This one's found in Majula. Head to the narrow path between the two tents and follow it. At its top is a large stone tablet; you can kneel before this to join the covenant.
The Company of Champions is very different from other covenants, in that it's meant for single player. Entering the covenant disables co-op and invasion play, and it increases all damage taken. It's essentially a "hard mode." While part of the covenant, invading NPC phantoms will drop Awestones upon defeat; normal enemies may occasionally drop them too. These can be returned to the tablet where you joined the covenant.
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