The Death Equation is a powerful magical effect capable of killing a deity and granting their domains to the user. Vecna somehow gained access to it (he is the god of secrets after all), and shared it with Tiamat in exchange for some unknown favour. The fact that he was willing to do this implies that he is somehow immune and the fact that he has yet to use it on Ioun implies that she is somehow immune. Delvash later tortured the secret out of Tiamat and, since he later used it on her, he is the only deity actually involved in the affairs of the material plane who knows how to kill another.
Using The Death Equation exerts a great toll on the user, enough so that after a single use Tiamat was so weak that Delvash could simply drag her to his lair and tie her to a rock. Subsequent uses use even more energy than the initial use (it is hard to tell how much, as only one being has ever used The Death Equation twice).