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.