Mortiferous is an adjective that refers to causing death or fatal injury. There are a range of synonyms that can be used to describe a situation or object that is mortiferous. Some possible options include lethal, deadly, fatal, life-threatening, terminal, destructive, dangerous, noxious, poisonous, and pestilent. Each of these words has a slightly different connotation, but all convey a sense of danger and potential harm. It is important to choose the right synonym depending on the context and intended meaning of the sentence. Using synonyms effectively can help add variety to writing and make it more engaging for readers.