There's a real-world thing called a Pata. It looks like this:
It is, more or less, a broadsword or rapier blade protruding from the end of an armoured gauntlet.
Quoth the wikipedia page,
The pata is most commonly paired with either a shield or another pata, though it can also be used with a javelin, axe, or belt. The restrictive handle was particularly suited to the stiff-wristed style of South Asian swordsmanship. Despite its shape, the pata is used primarily for cutting rather than thrusting. The extended grip provided by the forearm permits powerful slashes but restricts any thrusts. This can be seen in mardani khel today and in colonial descriptions which describe spinning techniques with dual pata "much like a windmill".
I wouldn't want to use one myself, but as they were used in anger in actual warfare it would appear that they are entirely practical weapons. It would be most useful for youre character if the fighting style they used before the accident matched well with the new weapon; this is probably not the case for many styles of longsword combat, but that's OK because you get to decide on the history here.
These weapons might be less effective against well armoured opponents... given that they're not so useful for thrusting attacks, even decent chainmail could provide an adequate defense, and there's clearly no scope for half-swording against opponents in a decent suit of plate armour. Of course, if your fictional weapon can transmute itself into any form that's not going to be a problem for your scenario.
I've had a quick look for a video of these things being used, but there's not a whole lot out there. When searching, do make sure you're looking for things with stiff blades as there are a lot of things which people call patas or dandpattas which are actually weirder things like the urumi (a kind of whip sword) with a gauntlet hilt. If anyone can find a good video of one of these things being used, please do share!