The phrase "once-powerful" can be replaced with a number of synonyms to convey the same meaning. Some options might include "formerly influential," "previously dominant," "past potent," "erstwhile mighty," "one-time forceful," or "former strong." Each of these phrases implies a similar idea: that something or someone was once in a position of strength or authority, but has since lost that power or influence. Synonyms can help to add variety to writing or speech, while also allowing the writer or speaker to more precisely convey the intended meaning.