Why do they need the Mortal Cup, again?

[Alyssa]

Sorry, but if anyone remembers - I just forgot - why do Jace and Clary need to find the mortal cup, again?

[Line]

Can't remember if this contains spoilers or not, so better hide it...

(view spoiler)[Because Valentine needs it, and the other mortal instruments, to call upon the angel (can't remember it's name), in order to make his wish and become 'all-powerfull'... (hide spoiler)]

[Nikita]

The Mortal Cup is important because it creates new Shadowhunters. If someone were to drink from the cup then they would become a Shadowhunter, however, it's dangerous because most don't survive the process.

Valentine wants it so he can create a new race of Shadowhunters. That's why Jace and Clary needed to find the moral cup, to stop Valentine from creating new Shadowhunters that would kill people who don't survive the process.

Hope this helps.

[Line]

Nikita: Isn't that just what they think he needs it for?

[Sharon]

Not sure, because Hodge was the one who explained it and he was working for Valentine. But I think that's what he actually needed it for. I forgot whether Valentine explained his real reason himself to Clary.