I still feel like cultists are one of the better values across all the chaos books. They have their challenges, but at least can help offset some of the more expensive point sinks of other CSM units, especially if compared to their loyalist counterpart.
I run both shooty and assault cultists as a mob pushing forward with my first units. The shooty guys just to throw out some dice which have enough volume to get lucky enough to cause a few wounds or kill a few models, and the assault guys to take the incoming eat or add some volume of dice in the assault wit auto pistol shots before charging.
Shooty ones always do well- get into cover helps no armor and t3 and auto guns are pretty good in mass.
Assault cultsits are weak, but I need the theme- they are never in cover, always moving, short range and t3.
Still good units, and they have a place even if not in horde mode.
Cultists of Chaos, the GODS are watching.
Immortality is yours!
I understand the theme part, I'll be more a fluff player when I start out, playing with friends so my two armies need to be more accurate lore wise than something twisted to give me wins. Sadly the way codex's are set out don't have unit limits on things like Heldrakes, GW leave that to us
Since you're going mass cultist, I guess you'll have tons of points to spare am I correct?
If the above statement are considered correct, I guess I'll consider the follow options and I'll explain why the crazyiness:
Now some of these options don't seem competitive sure, but if I think fun these are my first options personally. The deadly part, sorry, you might have me dead to rights on that but maybe something talks to you here inspiring you to a better idea with all the years of experience on the tabletop that I couldn't see.
-Chosen because I love them to bits for no damn reason. I'm still waiting for my Chaos Legions book to arrive so I have no idea how they've been buffed. I would love to use these guys in some melee way, but that requires tons of buffering which your cultists may just provide. Even if only two chosen hold on to some melta's/plasma and the rest are made for the fight it could be fun, but you'll burn cultist's faster and I'm not sure if you'll be able to bust through his lines with a few groups of chosen protected by cultists. Might need something extra for the punch in support of these blokes from what I read so far.
-Now the orphan, the guys I cannot defend with single shred of evidence. I do hope Traitor Legions helped these guys out cause they're so redundant at the moment, but oh my sack they are beautiful. I would love to figure out some strange way to make them work and maybe mass cultist could help out a bit: The warptalons. If you DS them in the enemy have to deal with them, this might save some cultists if you roll lucky on the blind, almost like a pre-chosen charge support. Blind them and run in with the melee cultist or chosen/berserkers/possesed/ect.. The real problem here is effectiveness and cost, will you have enough points to invest for such a tactic should it be plausible? Well, if so, I can promise the fun but not the effectiveness.I personally am gonna mess around with these guys just for the fun of it, because I had to choose the worst unit in the codex to love the most.
-I've seen some nice helbrute formations that might still fill nicely into the fluff. For some reason they look very fun to play with and they can take a small measure of punch which makes it a nice choice all round. It's as if helbrutes are made for cultists, I won't even be surprised if some cultists feel the need to marry one (good luck witht he powerfist on wedding night). I might even try to get a few of these blokes and mesh them up with some cultists sometime, it looks like some fun.
-I had to put in terminators because of my bias love for them. My heart can't accept the fact that the emperor's terminators are better than mine and I never will no matter how many engagements they lose against them. If all other reasoning fails, mine have tusks so the emperium can suck it. These guys will at least take the fire off your cultists AND give you the punch warptalons can't so it's a best of both worlds scenario since I think they're actually cheaper than warps with close to twice the points. either way, I always need terminators somewhere. I can't say how to kit them, but that's not the point, you've got terminators and that's all that matter in my books XD
I didn't put possesed in here because I still need to make up my mind on them and especially the amount of sculpting I'll have to do when I choose to field them. The way they look in the picture of the codex and what I get when I buy them form GW... really throws off my choice a bit. I still need to read through their stats, so I can't have an opinion on them except that their fugly if you buy them in my opinion.
Anycase, I just wanted to post some newbie thoughts and see if you can shed some light on my own choices as well.