Appearing to be but a simple decorated greatsword. The steel needing a bit of a polish. But the edge keen and a large emerald set into the cross guard. If inspected, a PC would find out that the blade is magical. After some time though, the wielder will start to hear a deep male voice in their head.
Balthazar tends to think aloud, his observations can be quite useful. Ever the pragmatist, his objective opinions are (thankfully) only heard by those attuned to him. His cursed exile has worn greatly on his manners, so he isn't all that polite.
For all intents and purposes, Balthazar is a sentient object as well as weapon and is indestructible. Despite this, he grumbles and gets upset at those that use him recklessly. Also, Balthazar is a capable spellcaster, willing to aid the heroes so long as they are respectul.
"You call yourself a hero? They must have lowered the standards since my days."
"Please don't throw me again, it's quite rude."
The Bladebound may very well be a great way to suggest certain courses of action and provide the players feedback and lore.