First of all, it's a samurai, and samurai have Bushido. So this katana-wielding Drudge Skeleton is a bit more skilled in a fight, and less likely to be blocked.
Second of all, its flavor is that of an evil samurai enchanted with dark energies. It's not just your ordinary bushido-trained rat -- it's Kuro's Taken. And we all remember who Kuro is, right? That's right: a 9/9 legendary demon (Kuro, Pitlord). Kuro's Taken is a dark samurai fuelled with the demonic energies of a legendary demon, with some creepy phantom-tendrils to show for it. Check him out in the full art below:
One last thing about Kuro's Taken. It was actually initially sketched as a human samurai by artist Puddnhead, then later changed to a rat in the final, at the art team's request. Here's a sneak peek at that sketch: