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Mmmm I'm wanting to use Archaon, Is it worth?
From my little experience with him, yes.
One of the best warriors, awesome weapons and magical items, and a lvl 2 wizard!?
But remember, he needs to take out so much of the enemy army to make up his points cost.
It also depends on how many points you are using. 2000-2500? probably not as a DP or Dragon would do better as they are faster. 3000+ go for it!
Repainting Orks not sure how many points yet
Warriors of Chaos shelved until further notice
2500pts is the minimum you should really take him as it then allows you to have a decent army to back him up with. The great thing is that he fills so many slots that your army would normally need to pay for seperatly. He is your general, a level 2 spell caster, a semi decent battle standard bearer and he causes terror. Have him ride up to your enemy general, issue a challenge and watch the carnage.
Support him with some more spell casters though as he will often be in combat before you get many spells off otherwise, two level 2 at 2500pts and a level 4 at 3000pts and above.
Hefty price for a single mounted model. 41.25 msrp when there are more elaborate mounted models for 30. I know thats not what you are talking about really but...
When I consider using Archaon is when I am building custom lords/heros and the point costs start creeping up to "I might as well just grab Archaon."
I know that feeling all too well.
I think one of the biggest plusses I see with Archaeon (besides the above) are the buffs he gives to his attached unit. Attach him to the Swords of Chaos (min 5 Knights plus immune to psychology), and they also benefit from the Mark of Nurgle when with Archaeon. Give the Swords mark of Tzeench and they are become ItP, enemy -1 WS in base contact, -1 to hit in shooting as well as a 6+ ward save. You still can give them a magic banner or such to plus them up further. Now THAT'S a Death Star.