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Does a Farseer need to disembark from his wave serpent in order to use a psychic power like Doom or Guide? (I would have thought 'yes', but this is based on the fire point rule on p66 which may just cover psychic shooting attacks).
If not, is there any restriction on how fast the WS can move and still allow the Farseer to use his power? (If the above was 'no', I would have thought this one would be combat speed only, per passengers firing).
But I'd really be happy to be shown why I'm wrong.
Thankyou for your advice.
The answer to this is in the Eldar FAQ. You don't need to disembark to use Eldar psychic powers (except shooting ones!) because they do not require a lin-of-sight to be used.
I think I see how this works - the FAQ only specifies which powers are shooting powers, and the rest just use the p50 rules, which say nothing about being a passenger, the movement/damage status of the vehicle etc.
As they do not require line of sight, it IS possible to buff units inside the same transport as the Farseer and any units outside of the Farseer's transport. However, it is not legal to cast guide or fortune on a unit that is in a near by (6" away) transport as the 5th ed FAQ clearly mentions you cannot target units inside a transport w/ psychic powers unless the psyker is in the same transport.
You also have to cast Guide, Doom or Fortune at the beginning of your turn, so moving the Vehicle doesn't even apply. You can't cast these powers after you move.