Cera Sumat, Holy Avenger is an item obtainable from any mob with a 1:15000 drop chance.
It is Legendary true melee sword, which means that after beating each vanilla boss its stats will go stronger. After entering hardmode, each hit would create small explosion, dealing part of the sword's damage.
All attacks are inflicting Curse of Light debuff. This debuff is making enemies to take 20% more damage from any sources. Also, affected enemies would deal 25% less damage to players.
It is also used in the recipe for the The Light of Cera Sumat.