Longsword of the Dawn is one of the big weapons in the game. While it is rather useless when you find it early, its one of the prime bosskillers out there if it doesnt pick up something very nasty on levelling. More damage than Thancs and even Sting, more pleasant to level up than Mormegil, none of the "curses" that come with most other contenders like anti-magic shell or mana drain unless you get unlucky.
But its striktly a swap in, youll need a workable blade for everyday use.