Underlord Holt works as one of the Emperor's Underlords. He ambushed and stole all of the gangs most valuable resources, including their slugs.
He makes his first appearance in Slugterra: Eastern Caverns, where he arrives at Serenity Cavern stealing all of the Slugs of The Outlaws' and the Shane Gang, including the Elemental Slugs.
He later reappears, in Get Pronto, taking command of a town run over by thieves under the command of the Dai-Fu and commands them to capture the Pronto and his friends.
In The Lady and the Sword, Dai-Fu manipulates Holt into controlling the Iron Warrior to trap the Shane Gang.
He also appears in The Emperor Strikes Back, imprisoned in a cell, for commiting treason, near the Shane Gang. He is also slugged by Swick before he frees the Shane Gang.