if you take a keep that is red/purp (lvl 7+) you get a bunch of keys that you can use at the chests downstairs. You randomly either get a bunch of glass, scales, BP tokens, or realm potions. You also get some decent RPs and BPs for killing the lord; think it's about 4k RPs and 600 BPs.
Holding a keep gives you a 10% damage bonus in terms of variance - basically brings your damage closer to cap like a huge str/dex/acuity buff. Putting a relic in it increases that amount for either melee or magic by an additional 10%.
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NF: Astlaine, Eleva, Cleanshot
CV: Brokinte, Busterbear, Fligoo
Mordred: Who cares?