Latent Effect is unlocked by using 13 weapon skills. Once you use 13 weapon skills, you will be able to use Final Heaven ONCE... to use it again, you need to do another 13 weapon skills.
Taipan Fangs +2 > Heofon Knuckles.
However they are a decent Weapon and faster to obtain. But please, don't consider them the "High-End" weapon, go for Taipan+2 at the least.
Hopefully latent effect won't be something absurd or near impossible to accomplish.
Positive notes: double light double dark skillchains.
Negative note: will never be a real relic.
Side note : all notes and denotes are not noted.