Formalcraft is a standard Arts CE buff, which is an increase in NP gain, damage and star generation for Arts cards. It is very versatile, as it can work on pretty much any servant with a decent amount of Arts cards.
Increase Arts Card effectiveness by 25%
Increase Arts Card effectiveness by 30%
How to use
Keep in mind that there are also other CEs that give a combination of an Arts effectiveness buff and another effect (such as critical damage, or NP damage) which might be more effective in some situations. Such as: Another Ending, Kaleido Sapphire.