Annihilus is a Unique Small Charm in Diablo 2 Resurrected (D2R). This Charm has a quality level of 110. This item should be kept in the inventory to gain bonuses. There are 3 important benefits coming from Annihilus:
Experience gained from killing monsters is increased.
The character will be more resistant to any elemental damage.
Gains extra skill points to all of the character's spells and abilities.
The fastest way to get this item on Battle.net is via trading, you can use the Item Price Checker to see how much Annihilus is worth.
Another way of getting this item is by killing the Diablo Clone.