At some point, Scar recruited Sumu part of his plan to eliminate both Simba and the Lion Guard in the same day. First he has the scorpion sting Simba as they celebrated his defeat over Scar. In order to save his life, the Lion Guard must retrieve the only known cure for scorpion venom: volcanic ash, which the nearest source is the volcano in the Outlands. This allowed Scar to set up an ambush for the Lion Guard with his minions so that they can destroy them all at once on their home turf. Alas, the plan failed when the Lion Guard escaped and managed to cure Simba.
Sumu is the first insect to be an official character in the Lion King series, and is also capable of speech, which is unusual considering most other insects are considered non-sapient by Lion King standards.
Sumu is the third venomous villain in The Lion Guard, the first two being Ushari and Kenge.