Shadow Cutter is a rapier of which only Rose can use. It can be bought from the weapons store in Fletz for 200 gold. This weapon cannot be found in any chests or obtained by defeating enemies, and also is the only elemental weapon of which Rose will ever receive, dealing Darkness element damage.
This weapon will find little use for its Dark Element. There are no Light Element creatures until its base damage has been surpassed by the Dancing Dagger, and then it is only one, the Fairy, a somewhat uncommon attacker amongst hordes of others in Kadessa. So although the Shadow Cutter is good until the Phantom Ship, come the third disc when a better weapon is found, it will as a rule cease to be useful even with its elemental bonus. Details such as Overkill matter, though, and if the Shadow Cutter still kills other enemies in the same number of hits as the Dancing Dagger, and kills Fairies faster, then it is still better. So it can either be assumed that it is worse, or it must be tested. At least we can be sure that there are no Dark Element enemies in the Forbidden Land. Getting more hits in on Fairies can be finessed by hitting other targets with your other team members, and leaving Fairies for Rose to finish off. Watch out for spamming spellcasters with red health indicators though.
After the first half of the second disc Dancing Dagger can be found in the chest after the Ghost Commander at the Phantom Ship, or, arriving at Furni at the beginning of the third disc it, it can be purchased.