Two centuries ago, Thardoss, Chimera Lord of the windswept Isles comissioned a weapon to be crafted in the likeness of his ever-changing form. The weapon was known as 'Thardoss' Gift' or more popularly, 'Quicksilver,' after the metal with which it was forged. Thardoss hired a spellsmith to imbue the weapon with great strength and tempered it with his own flame and blood. The result was a beautiful longsword, that was hard as steel and the magic within giving it an ever keen edge.
'Thardoss' Gift' was decades after it's creation, given to the diplomat Cyrus Saldamor as a sign of respect. The weapon remained a Saldamor heirloom for three generations until 'Thardoss' Gift' was lost in the war against the Szael empire.
The weapon changes form according to the needs of the wielding individual. The sword when not in posession of a sentient entity, the chimeric blade appears as an ordinary +1 longsword and is always treated as a magic melee weapon. Need a mace to bash an opponents shield? You got it. A spear to pierce the thick hide of that Dire beast? It's already in your hands! An axe? Dagger? Any true weapon simple in form is always at your side.
The Chimeric Blade seems to grow more effective the longer you fight with it. But actually is attuning it's magic to it's new form. Starting at -2, and increasing each round by 1 to a maximum effectiveness of +3.
Thardoss' Gift, being quicksilver, has an effect unaccounted for by it's creators. Prolonged exposure to this weapon will wear on the sanity of those in close proximity. Delirium and death inevitably befall those who use it without caution.
Can be detected as magical, but it's nature is revealed based upon whatever enemies the wielder is facing at the moment.