In the last session that I ran when the party was clearing a dungeon they came upon a room with a couple of dreugar in it and the Bard decided to use Major Image to make it look like the floor had collapsed and the dreugar were now falling. I had the dreugar make an int save which they failed and gave the party advantage on attacks against them since they were failing around thinking they were falling. I did make the bard use his action on subsequent turns to maintain the illusion. 

Was that the correct way to run it or should I have told bard that the dreugar would immediately noticed that the floor was not falling the spell was wasted?

If so any suggestions on what the bard could have done with major image instead? The room was 20 by 20 with two entrances with some mining equipment in it.

Also can someone who casts an illusion allow a creature to see through the illusion?

Thanks for reading.

Yeah, they probably should have seen through that right away. Remember that physical interaction with an illusion immediately reveals to the target that it's an illusion, and it becomes semi-transparent to them. So while the Bard could have used Major Image to make the room look like it's collapsing, once the actual empty ground reached the targets the fact that they still felt their feet on solid ground would reveal to them the illusion. As well, the illusion only produces sight, sound, etc... but no shaking of the earth. With a low enough INT check I think it would be fair to ignore that, and maybe your Bard could have forced them all to bunch up in a corner, but the moment they were standing over what they thought was empty ground they should have seen through the illusion.

As for allowing a creature to see through the illusion... short of having the character inform the other characters ahead of time that it's an illusion, I don't really know what else could clue them in.

Thanks for the info, I'll let them know during the start of the next session.

I use major Image a lot... my favorite trick is to use it create ghosts. That way if the illusion passes through objects or if the target interacts with it the fact that it seems ethereal doesn't necessarily break the illusion.

That is a really good idea.

I think I'll use this to just describe some of the weirder uses of Major Image I've done just for funsies, and I encourage anyone else to do the same.

Once I managed to clear out an entire guard's station by creating the image of of ghost girl who resembled a young girl we knew was murdered in town. She did weird The Ring style stuff (spider-crawling, twitching weirdly, rushing unnaturally fast at guards). She even moved outside of my visual range into the building itself, but since she appeared to be a ghost it actually made it more effective that she just passed through objects. In the end she was "destroyed' when she touched a magic-cancelling noose used for executing magic users, destroying the noose in the process (which we had actually stolen earlier and this was just to keep the guards from wondering where the noose went)

Another time I occupied and frightened an entire group of city guards (we had a lot of conflict with guards) by creating a Major Image of a large demonic dragon. I ran around hidden under the image, knocking over objects using Thunderwave and generally creating a mess. When we needed the deception to end I just made it look like one of the crossbow bolts they fired hit it in its "evil eye", and then the illusion erupted into flame while proclaiming it would return in 100 years to take vengeance on his descendants.

And, of course... the classic convincing zealots that their God was manifested in front of them to keep them distracted while the rest of the party bum rushed the cult leader. My DM was nice enough to extend my Actor feat to let it apply to this deception as well.

Those are some really great ways to use Major image! I'll make sure to show the bard this post

Thanks! I'm hoping some other users offer some stories about their uses of Major Image. It's a spell that really relies on the creativity of the user, so it's always fun to hear ways that it was used... even massive failures can be fun to hear about.

Quote from Neo3602 >>

In the last session that I ran when the party was clearing a dungeon they came upon a room with a couple of dreugar in it and the Bard decided to use Major Image to make it look like the floor had collapsed and the dreugar were now falling. I had the dreugar make an int save which they failed and gave the party advantage on attacks against them since they were failing around thinking they were falling. I did make the bard use his action on subsequent turns to maintain the illusion. 

Was that the correct way to run it or should I have told bard that the dreugar would immediately noticed that the floor was not falling the spell was wasted?

If so any suggestions on what the bard could have done with major image instead? The room was 20 by 20 with two entrances with some mining equipment in it.

Also can someone who casts an illusion allow a creature to see through the illusion?

Thanks for reading.

 Something like what he did should definitely be possible with magic, but I can't find a better spell than this (short of mirage arcane which is a way more than needed).

Some minor changes I would have made is make the illusion make the ground crumble and fall with them so that the creatures are falling with a chunk of ground. That way the "there is still a ground" issue isnt as bad. I would also give them advantage against the illusion after they see creatures walking on air to attack them.

I like to mix illusion spells with cantrips and non-concentration spells to boost their believability.  Thaumaturgy is a great for this, as are spells like Earth Tremor, Gust, Catapult and the aforementioned Thunderwave.  If part of the illusion Feels real, then its overall credibility goes up.