Pre Vizsla is the leader of Death Watch, introduced in the second season of Star Wars: The Clone Wars in 2010 and voiced by Jon Favreau — who would go on to create The Mandalorian nine years later. StarWars.com describes the double life precisely: he "served as governor of Concordia, a moon of Mandalore, and appeared to be a close ally of Duchess Satine Kryze", while secretly leading "a hidden group of Mandalorian commandos seeking control of their planet".
The Darksaber's first modern holder
He carries the Darksaber, and it is through him that the weapon enters the modern canon at all — a black-bladed lightsaber, made by the first Mandalorian Jedi, held by his descendant and used as a claim to rule. Almost the entire Mandalorian storyline of the last fifteen years, across three television series, runs off that one prop.
Alliances that eat him
He allies with Count Dooku and the Separatists to bring down Satine Kryze's pacifist government, and when that partnership sours he takes in a shipwrecked Maul and commits Death Watch to the Shadow Collective. It works: they take Mandalore. Then Maul challenges him to single combat under Mandalorian law, wins, and beheads him — and the tradition that legitimised Vizsla legitimises his killer.
Why the character lands
Vizsla is a genuine political actor rather than a thug. He believes Mandalore has been hollowed out by pacifism, he is not wrong that the New Mandalorians are fragile, and his methods are indefensible. The show lets all three of those be true at once, which is more nuance than a children's cartoon is generally allowed.