This dagger gives a hidden 5 bonus for thieving, so if you attack an enemy with the dagger equipped and you have 35 thieving you'll be able to steal from them. The seal is destroyed once you've acquired the item and killed Zamorak. If Azzanadra's Seal is available an imp could grant you a brand new seal if the dagger is lost.
You'll get black sword fragments if you summon a lesser Demon. More Demon invoking will give you red sword chunks. K'ril Tsutsaroth's invocation will provide you with demonic blade plans as well as an hilt of the sword. The Demon Butler (it's item name is Alathazdrar's Seal but the look-up information states it's the demon Butler's) When invoked, it will take you to Sergeant Damien. Invoking once more will grant you 200 curry bowls.