Erebus, the first dark apostle, destinies hand, corrupter of Horus Lupercal
At the price of 195 search lights Erebus appears to have a low statline, nothing overly remarkable as a CCQ character. This is how ever not his role in your army. Erebus is both a Diabolist -giving you access to Dark Channeling- and a Chaplin -filling the extra character requirement-.
Erebus allows you to take demon allies without needing to take the special rite of war, while he would instantly fill the requirements for it, this allows you to take an orbital strike or angels wrath or pride of the legion with Demons. This is great in larger point games, giving you mass Tzenntch summoning or Khorne ap3 juggernauts and bloodletters.
As a burning law psyker, Erebus has access to either biomancy or telepathy. Biomancy can bring him to strength 9 with smash (+3 for the power, +2 for the crozius) giving him ap2 or he can get 7 attacks on the charge (+3 from the power and +1 for charging) at strength 6. -1 toughness for enemy units makes tactical squads very tasty and the relentless 4+ FNP makes your tacticals total beasts: Rapid fire charge with Dark channeling for rerolls, strenght 5 or fear and 41 attacks (you are taking additional close comabt weapons right?) Combined with Adamatium will Erebus actually gives his squad 4+ Deny The Witch against mastery level 1.
Other biomancy witchfires are pretty neat with ap2, one of which can heal your unit (see my unit combo later for how i use this, but generally heal himself or near by HQs).
Telepathy has one of the best Primaris powers: psychic shriek. Rolling on Telepathy is mostly going to make your enemy weaker: LD checks to fire overwatch, (amazing when it goes off) move, shoot and counter charge you after your unit disembarks. Telepathy also has a power which lowers enemy units Leadership, but this power risks the unit breaking morale and running away from your assault ready units. This power is best used on heavy support squads, and other back field infantry units who are close to the board edge. This power can be used to make expensive squads to run off the battle field, costing your opponent those units and wasting those points. Regular opponents will then deploy further away from the board edge to prevent this, which only brings them closer to your assault army.
With 3 wounds and artificer armour you can use Erebus to potenitally tank ap3 wounds from the squad, this is risky but can pay off in an expensive squad.
By putting Erebus in a squad, the sqaud no longer benifits from 50% of the Dark Channeling table (hatred and fearless (zealot)) as he gives out that rule passively. In the veteran squad I run Erebus with they also have furious charge, negating the dark channeling result. I dont rate the demon 5++ and fear rule as they count as destroyed as the end of the game and most importantly they no longer score.
I run Erebus in a Phobos landraider (the only assault land raider) with a veteran squad with power swords and furious charge (dark channeling is no longer need in my opinion so i give them all melta bombs instead) and Hol Beloth. This unit is a bit of a death star but it only comes with standard 3+ armour. I use this unit to ERASE power armoured units and warlord units. Do not ever use this unit agaist 2+ amour, ever.
Thats my take on Erebus of the Word Bearers. Thanks for reading.