What is it ?

A mod adding a new illager boss/subboss with its own magic that you can use if defeated.

What does it offer ?

A necromancer found in his tower, which serves as a powerful foe to beat as he can shoot dark lasers or generate a field of withering darkness around him; he also can raise the dead around you, surrounding you with enemies while you are still trying to beat him. It seems the boss can regenerate somewhat (maybe he just has ridiculous hp), which means you really have to get hyper-focused on attacking him over the hordes he invokes.
After beating him he might give you his staff/armor pieces which grants you abilities such as summoning zombies and skeletons, getting some undead to your side, prevent undead aggro, grant sun immunity to close enough mobs and making your resistant to some potion effects at the cost of making you immune to the regeneration effect too.

I found that the necromancer boss fight (if fought within the context of the overly-frequent tower he spawns in) is almost too hard as it is supposed to occur within a space too tight considering his main gimmick is summoning hordes of zombies and skeletons to distract you from dealing damage to him (maybe putting stairs instead of a ladder as access to the roof may have at least offered an easy way to lure him there instead being doomed to fight in an enclosed room ? technically breaking the floor is still an option I know but still, it feels like it wasn't very well thought out). Always remember that regen/enemy spam makes for the worst bosses.

Also when I say the tower is overly frequent look at the screenshot its crazy.

In conclusion

This mod while having some potential for a simple boss mod seems more like a quick test which was never meant to be taken seriously by anyone, just another Halloween mod meant to be played once before being forgotten.
Unfortunately the mod seems already abandoned and won't receive any more 1.12 updates, which means bugfixes and new content won't get added.
I would like to mention however that a fork/port of this mod is present in 1.20.1 under the name Necromancer Mod Legacy(Raid!) ; I've played that 1.20.1 version of the mod most and might even write a separate review for it if it gets new content or if the texture issues unique to this port gets fixed (at least this mob works better within the context of raids than its original structure).

There's sadly no config file which means the mod can't support modded mobs for some of the custom armor's features (unless other mods go out of their way to add compatibility with this specific niche mod), or that there's no way to make the mod's structure rarer/disable it (except maybe by using recurrent complex's config file ? ' Don't know I have not tried)