It's a difficult Weapon Aura to utilise, because it's entirely reliant on the weapon "core" being a complimentary colour. Which is why the sickle that you are given as well has that innate green aura in the middle of the weapon.
Here we have Barovian Mists and Necro Aura on a Tabaxi staff.
Here, we have it on some Sunswords, Feywild Illusory Shadow and Neco.
Sanctify with Enhanced Acid
Now with Necro (and you can see how you lose an innate internal aura, meaning dark weapons or weapons with a dark material base are going to have a rough time with this Aura)
And Blackrazor with Necro and Enhanced Acid.
Out of all the Auras, this is easily one of the more difficult ones to use.
J1NG