World Eaters have specific and strict guidelines as to what should be in the army list. Those are:
- Units should be Khorne marked or have the ability to be Khorne marked wherever possible.
- No magical spells or abilities can be used.
- All World Eaters are "Berzerkers."
Given the above guidelines it is easy to see that there are a lot of units that cannot be used from the Chaos Space Marine codex to strictly play a World Eaters army. What also comes a long with that is a deficiency in adaptability. For example, in an army where all models were "Berzerkers" then almost no melta guns would be available.
A quick note on "All World Eaters are Berzerkers". This is some fluff from long ago and some may construe it that only Khorne Berzerkers can be used. To me this is not the case, but it means that all World Eaters have had the psycho-surgery to enhance aggressiveness that the codex mentions.
The following list are units in the Chaos Space Marine codex that, in my opinion, are acceptable to use in a strict World Eaters Army.
Kharn the Betrayer
Demon Prince (Khorne Marked)
Chaos Lord (Khorne Marked)
Chaos Terminator Lord (Khorne Marked)
Summoned Greater Demon
Chaos Space Marines
Summoned Lesser Demons
Using the units above can make for a pretty limited list, so in some cases some allied units could be acceptable in a non-strict world eaters army. These are:
Abaddon the Despoiler
Lots of fun can be had with a World Eaters army when restricted to the above units, but thanks to the limited dimensions of the codex writers not much thought was put into lots of aspects of the units. For example, a Khorne marked Demon Prince pretty much sucks compared to the abilities of the other marked Demon Princes; the same can be said for the Chaos Lord. Oooh, cool, +1 attack! Pffft! I think being Khorne Marked should at least impart +1 WS, +1 Attack, and Furious Charge. Some options to buy might be blessing of the blood god or something a little better than just +1 attach while others get to use powers that are exceptional force multipliers (lash of submission).
That said, I think for those of use who love World Eaters and want to be competitive at the same time that it should be acceptable to use stretched fluff to incorporate some of the better and stronger aspects of the codex into a World Eaters army.
Demon Prince Marks & Psychic Powers
The Demon prince is probably the best generic HQ in the whole game thanks to the marks and Chaos's psychic powers. Plus one attack just doesn't cut it for the mark of Khorne, and unmakred prince using warptime is probably better.
Mark of Nurgle imparts +1 toughness making the prince hard to kill when receiving small arms fire. However, given that he is a large target and monsterous creatures are best killed by heavy weapons fire, if adequate cover is not constantly provided the extra toughness can be a waste.
Mark of Tzeetnch imparts a 4+ invulnerable save. This means that the demon prince is "in cover" no matter where he goes and is better able to thwart incoming heavy weapons fire.
Mark of Slaanesh imparts +1 initiative. To me this rarely comes in handy but this mark is required for the use of the lash of submission psychic power.
Lash of submission is one of the best, overused, and anticipated psychic powers int he game allowing the player to move the enemy's models anywhere within 2d6". Grouping models together and using templates on them is a devastating combo. However, thanks to its over use and other armies recently gaining outstanding psychic abilities, other armies have enhanced their psychic defenses making perils of the warp and the power being unreliable is a real possibility. Lash of submission or any other psychic power should never be relied upon to win the game.
Given those facts, and combined with the Demon Prince's size, I think a Chaos Sorcerer has some potential. A sorcerer comes with a force weapon and can be given marks, wings, terminator armor, plasma pistol, and, with a familiar, the use of two psychic powers. However, the cost could get pretty high with all of those options.
New units in the game have high toughness or exceptional special abilities (New Blood Angels & Tyranids) but are lacking eternal warrior. The use of force weapons and the Bliss Giver demon weapon are going to be seeing more use. A Chaos Sorcerer with warptime and the force weapon's abilities is going to be a devastating combo to a lot of these new units.
One consideration is to use a Sorcerer with familiar, warptime, and lash of submission. The warptime and force weapon can be used in close combat to eliminate troublesome characters and the added +1 initiative can really help in this situation. Lash of Submission is just a bonus.
Another consideration is the gift of chaos. No matter the toughness or if a character has eternal warrior they could be turned into a blob of flesh or a spawn on a good roll of the die. And with a familiar the power can be cast twice. The downside is the power must be cast at the beginning of the turn and higher toughness models can only be 'spawned' on a roll of 6.
Incorporating Psychic powers into a World Eaters Army
If a Sorcerer of non Khorne marked demon prince is used then the World Eaters army isn't really a strict World Eaters army. The conundrum of using psychic powers in a World Eaters army by stretching the fluff.
Warptime can be considered as the demon prince harnessing the rage of Khorne to his advantage. Lash of Submission can be considered as a Blood Thirster using the whip me holds in his hand. A poster on DakkaDakka suggested using a tech marine as a sorcerer and lash of submission is a old technology gravity gun modified to suck enemies closer to ensure a devastating assault.
Whatever fluff is made up there must be a differentiation between strict World Eaters, World Eaters allied with other units, and Competitive World Eaters.