Voice of Twilight does not target any units, if it is cast from inside a Wave Serpent does it only affect the embarked unit or all units within 12"?
And that took bugging multiple Eldar for the wording as it's not in the main Codex, and apparently paying for another full Codex price just to get Iyanden isn't that popular around these parts. As it thus is targeting instead of 'saying otherwise', the Psyker runs into the Declare Target rules on Page 67 and can only target himself, his vehicle, or another unit in the same transport.A blessing that targets all friendly units inside 12"
Do note that this means Psyker powers can be targeted at the Wave Serpent itself, whether you're outside or inside it. Given the nature of most Psyker powers this is of limited relevance, but it can potentially work miracles with powers like Forewarning, Fire Shield, or in the case of Eldar the ever useful FortuneSimilarly, a Psyker outside a transport cannot target a unit embarked within one.