This question already has an answer here:
Yesterday I was running the 5th Edition Hoard of the Dragon Queen adventure and one of the players was separated from the party. He entered a room in the dungeon and saw three blue guard drakes as well as several kobolds. These guard drakes were younger and only had 33 hitpoints each as opposed to the normal 52, but still had normal attacks. This player had 24 hitpoints at this time and I just ran some tests and he would have died in one round around 57% of the time, not including damage from kobolds.
I showed him a picture of the creature, I described the size of the creature and told him there were three. But he still seemed determined to jump into the pit to fight them. He definitively told me that he would like to jump into the pit with these creatures. I felt that this was my fault as I didn't describe how dangerous this encounter was, so I pulled punches behind the screen and had them bring him to only 4 hp instead of killing him and let him escape the pit and return to the party.
Looking back on this the only other option I had would have been to say something similar to "The total hitpoints of these creatures is around 100" but simply telling statistics would ruin immersion.
Is there a way to accurately describe how dangerous a creature is without simply giving away its statistics?