Theis rapier is one of my fav weapons for both PvE and PvP. Fast, unpredictable movements + quick, damaging strikes with decent reach, and it has a block breaking attack that will often do 500 - 600 damage through shields. This can open up the enemy for a deadly counter. Pair with Pontiff's Right Eye and/or Hornet Ring. The critical damage is in the 1000 hp range and will one hit kill many players on a backstab.
It is particularly effective against certain bosses, especially the trickier ones like Lothric and Lorian or Soul of Cinder. With Pontiff's Right Eye and Carthus Beacon, SoC was a pushover. The range is enough to reach Lothric when he is riding on his brother's back, allowing you to viciously punish him with strong attacks or a flurry of light attacks. You can use the Weapon Art to move around too. Good for avoiding stuff as a last ditch attempt to get out of somewhere.
The frost buildup seems not to do very much. I've only seen it kick in once or twice, total, and I've used this thing for two complete NG+ runs now. Hypothetically, it should be really good, as it causes the enemy to take more damage and slows down stamina regen. It just never seems to actually happen.