[Gregstrom]

Mind Hunt: Pointless against Astral mages, and thus pretty risky in the late game.

Seeking Arrow: Kills a 10 HP mage ~1/3 of the time. Large numbers of mages dilute the effect of spamming it.

Earth Attack: Very nasty attacker, but E5 mages can be hard to come by.

Ashen Angel: Risks suicide if you use against one or two commanders, but many consider it the most dangerous assassin spell. If you've bootstrapped Death to the point where you have multiple demiliches, it's a good choice.

Infernal Disease: Not the most dangerous of the spells, but if you're a Blood nation you may well be able to spam these in terrifying numbers towards the endgame. And of course even a failed attack may well disease the target, causing a slow and lingering death.

Send Tupilak: LA Atlantis only. Cheap and accessible for their mages, so it should be spammable. May be less effective against thugs than other assassin spells.

Flames from Afar: Only likely to be effective against large groups of researchers.

Flames from the Sky: Effective against most non-giants, but expensive.

Murdering Winter/Wolven Winter combo: Effective but expensive again.

A couple of less obvious options:

Rain of Blood: 5% of diseasing any mage in the province. Also an outside chance that the added Misfortune will trigger a nasty event.

Leprosy: 50% of mages in the province get diseased.