Setup: Centuries before a group of mages known as the "Lords" created a magical guardian to keep others away from an underground region of enormous power. The creature was a guardian naga. It was granted immortality, so long as it remained true to its purpose.
Fast forward to the present day. An evil cleric has acquired a powerful item which allows her to get past the naga by mimicking a Lord (the item was specifically crafted with this power). As part of the glamer used, the naga would also forget that the "Lord" had passed by. The cleric walks past the naga and, upon reaching the area beyond, begins an evil ritual - drawing on the power of the region.
The party (3rd/4th level, IIRC) were pursuing the cleric for reasons I forget. They had tracked her through the Underdark when they encountered the guardian naga. The naga naturally refused to allow them beyond, as only a "Lord" was allowed to pass. The party, realizing they were badly outclassed, showed discretion and attempted to find another way through the tunnels. Failing, they returned to the naga and began to question it.
Through some good questioning, they learned the naga's purpose. But they also knew that the cleric had gotten past. And the naga kept denying it, at first vehemently, then tentatively, then plaintively. As the debate continued, the party saw the naga actually begin to dissolve, in great emotional pain. Eventually, convinced that it had failed in its purpose, it crumbled completely and the party were able to reach the cleric and (barely) stop the ritual.
The naga encounter had no combat/magical/skill use solution. The players had to figure out what was going on, then convince the naga that it had failed. But in doing so, they caused considerable pain to a basically good being. It worked out well and was quite memorable.