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So since my High Elves are making their way back into AoS I'm considering giving it a shot. Been keeping tabs on AoS and have a general idea of the rules and what not and was wondering if you can cast spells on allied units. More to the point the new elf spell speed of hysh if I take gotrek as an ally can I cast it on him?

In the spell description, it will dictate what you may target with that spell. "Speed of Hysh" specifies that it targets 1 Friendly (your army) LUMINETH REALM-LORDS unit, wholly within 18"

The Lumineth Realm-Lords part, means that it may only target allies with the Lumineth Realm-Lords Keyword on their unit warscroll. Every model in the Lumineth army has this keyword. Gotrek does not. This prevents you from casting Speed of Hysh on Gotrek.

Edit: For clarities sake, if Speed of Hysh did not have the LUMINETH REALM-LORDS part in its rules, and only the Friendly portion, it would be allowed to be cast on Gotrek as an ally.

Basically aos version one was a broken mess, day one people found game breaking combos so buff spells tend to be restricted to units in the same army.

Sometimes like with cities of sigmar tempest keep you can include units from another book and they'll gain the keyword allowing you to use specific spells but that's an exception not the norm.