Surely it is the other way round, The ability to one phase of irresistible Lore of Heavens spells is the reason to take a Heavens Slann The comet reroll is only allowed to be used if Tetto eko has cast the spell, not if the slann has.
Ok so I played the game vs dwarves today and was surprised to find that he had not taken any warmachines whatsoever, which annoyed my 6 terradons and 12 chamo skinks (they still managed to earn their points mostly! ). What he did take was 4 hordes: GW warriors, GW hammerers, GW Ironbreakers and hand wep and shield Longbeards, with his invincible lord in the hammeres unit. I had taken a couple blocks of saurus, some skinks, a steg, the above mentioned warmachine hunters, tetto and a shadow slann (i felt too mean using heavens slann). The hammerers with lord was charged by one of my saurus blocks, and in the rear by my steg, I then cast Okkam's Mindrazor on the saurus, hexing the hammeres with an Irresistable Iceshard Blizzard from tetto, so after a couple turns of combat all that was left was the lord and his thane, who killed the saurus and steg. Then to my opponents horror the slann Irresitably cast Pendulum at the Lord and thane, killing them both after he failed his ward. So thats how you kill an Insane Dwarf Lord!
No, it works that the irresistible is only for Tetto, not the slann. If the slann gets the comet spell off, tetto allows the reroll of all Comets. I do not think that all heavens casters get the irresistible on doubles. Grats on the win too btw.
Herald of Cosmic Events says: ... All spells from the Lore of Heavens... Seems clear to me, any wizard with heavens benefits. The Stellar Staff says: If Tetto'eko successfully casts the spell... Also quite clear, only his comet benefits. Both rules are found on page 69 *insert immature joke here*.