After getting Kubfu to at least Level 30 (if you haven't completed the Pokemon Sword & Shield Story) or Level 70 (if you've already completed the Pokemon Sword & Shield Story), you will be able to challenge the Tower of Darkness if you so choose. Here, you will have to defeat five different trainers with only Kubfu in your party. The final trainer will be none other than Mustard, who will challenge you with a Kubfu of his own. Defeat all five trainers, including Mustard, to allow Kubfu to read the Darkness Scroll and evolve into Urshifu. Because you took on the Tower of Darkness, Urshifu will be in its Single Strike Form.
Master Dojo Student: Level 23/65 Zorua
Master Dojo Student: Level 24/66 Scraggy
Master Dojo Student: Level 25/67 Inkay
Master Dojo Student: Level 26/68 Krokorok