How can you get steiner;s last weapon? the excalibur? I heard that steiner can cast dark side and climhazzard!!!!!!!

You can get Darkside ability from the Blood Sword obtained from giving in a certain amount of Stellazzio Zodiac coins to the queen in Treno. Also, the Excalibur technically isn't Steiner's last weapon and is certainly not the strongest. Excalibur II is his final weapon, however due to the circumstance that you can only get it if you defeat Lich in Memoria within 12 hours, its unlikely that you can get this in a full play-through or without cheating.

Anyway you can get the Excalibur by at first talking to a man near the main Library part of Daguerreo he will want a key item "Magic Finger obtained from the Auction House in Treno.

To get this, you need to obtain all key items presented in the Auction House (e.g. Rat Tail, Une's Mirror etc) and sell them all to Nobles looking for them all around Treno. This will trigger the Magic Finger to pop up in the Auction. Obtain this item and give it to the old man in Daguerreo. In return he will give you the Excalibur.

On a sidenote, you only need the weapon for learning an ability. The Ragnarok obtained from a treasure chest during the Chocograph Sidequest is much stronger.