Merger Weapons commonly have requirements on items needed to merge for them or upgrade them, it all depends on the weapon.
with weapons
the doom weapons are level 8 weapons and do lots of damage.
U merge it w/ To Valencia and Never Ending Storybook
do the packing up quest (part of the fire war chain)
the three doom weapons i would say
you first need to reach chapter 6 in the frostvale story book and get into a place called breaking in. from there obtain coal merge coal with the items to form ur togsmas items.
In quests like defend the keep, sir jing's weapons and other ones
there are a lot of good weapons, check out this link:http://dfwiki.com/index.php/Best_of_the_Best
you can check it at Dragon Fable Wiki>Weapons>Weapons listings by level>Category:Level 16 Daggers
u download them at wpe pro or crash override
You buy it at Lim in the Falconreach weapons shop. You need a dragon amulet though.
There is zangetsu (in both shikai and bankai forms) and you can get them from cicero's shop on dragon coins :|