Updating Karthus means we can apply all the tech and knowhow that we’ve picked up since launching the game. For example, we’re actually going to animate his robes so he doesn’t look like a jellyfish in his idle state, and we’re building a whole range of Lay Waste animations so he Qs with variety. On top of that, we’re greatly increasing the fidelity of his model. Karthus looks a little different – we’ve made him less bare-bones – because we now have the tech and expertise needed to display the level of detail that a withered necromancer visually demands. In addition, the change means players around the world can enjoy the same awesome Karthus without us having to switch things up for various cultural sensibilities.
Source: Riot Games