This is probably one of the easiest boss battles in the game. Doesn't take much to beat it, but you still need to be careful.
A swarm of rats will appear once you enter the fog gate. You need to kill 10 for the actual boss to appear. Be wary, as the regular rats will continuously spawn until the actual boss is defeated.
Royal Rat Vanguard will appear, and although his attack patterns are exactly the same as the other rats, the damage he deals is stronger and will likely inflict poison on you if you get hit. Be sure to pack poison moss or other anti-poison items with you.
Defeating him will give you 'Royal Rat Vanguard Soul', 'Rat Tail' and 12,000 souls
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The Royal Rat Vanguard is a boss in Dark Souls II. The Royal Rat Vanguard is fought in the Grave...