Lord Gui Lang was the ambassador of the Underworld who allied with Heitaros Cratsus.
There is nothing else known about Gui Lang other than being a high ranked official in the Underworld.
Act 19.9: Duel, Appears!
The Grand Chase caught a glimpse of Lord Gui Lang set foot in the Underworld Train with the Hardliners simply occupying the front of the train, not doing anything to control it. They thought it was odd how he did not seem to care about what was going on in his homeland.
Epilogue 5.1: Ticket to Underworld
As the Grand Chase focused on the current situation they were in, Arme worried that Heitaros' evil influence might be all over the Underworld already. In the dark about the Ambassador, Lire was speculating that Heitaros and the Underworld may have developed a friendly relationship whereas Ronan thought the Demon God was tricking them by disguising himself before he attacks the Underworld.
Epilogue 5.2: Underworld Ambassador
Seeing what Rufus has done, Karp asked if the Underworld is now catching people who are alive with Gui Lang responding that it is how the Wilde Bounty Hunters live. He recognized the group to be those that got kicked out to the Wilderness so the Ambassador gave him a rundown, adding that the Underworld is in a peaceful reign because of the overflowing number of souls all thanks to Heitaros. Their conversation came to a halt as Hyde reported a problem that came up.
When Karp spotted the Grand Chase, he informed the Ambassador about them causing trouble after they entered the Demon World, therefore, they would do the same in the Underworld. Despite the noble's protest, Lord Gui Lang offered to deal with their enemies himself, ultimately ending to his defeat. Before perishing, he swore to never forget his grudge even in death.
Gui Lang can be fought as a boss in Epilogue 5.2: Underworld Ambassador.
The Ambassador receives less damage through the 'Power of Underworld' and attacks by duplicating himself. Lord Gui Lang and his clones deal area of effect damage over time. Then with 'Dark Wave', they chase after a party member. For every usage, 'Underworld Touch' stacks up and reduces the party's recovery rate.