A first event

Say you want a hidden door in a stone hallway. The player walks the last block, the wall opens, it never happens again.

In creative, put a cyan Event Trigger on that last floor block, or inject the id onto the stone itself. Put an orange Event Location in the wall you want to delete. Same id on both, something like mypack:hall_door.

Poll, data/mypack/command_poll/hall_door.json, one weighted command:

setblock ~ ~ ~ minecraft:air

Event, data/mypack/event_trigger/hall_door.json: walk_through, not persistent, command_location set to event_location, range 8. The ~ in the command is the orange block, not the player.

Reload, walk the hall in survival. The wall is gone. Walk it again and nothing happens. That is a one-shot.

Want a mob instead? Same setup, poll command is summon zombie ~ ~1 ~. The zombie appears on the marker, not in your face, if you used event_location.

Need this inside a dungeon generate? Prefer EpicQuest Dungeoning event markers for that. They reset when the instance wipes. EventMaker is the overworld / hub / static structure tool.