"Each mech has a special gem embedded in its body, underneath the chest plate. This is a failsafe. If the mech is destroyed, this gem detonates as a 20d6 fireball - a tremendous fiery cloud about 40 feet in diameter. This makes the automaton particularly dangerous as the explosion below deck might breach the ship's hull. If the player characters can somehow get it up on deck or off the ship before it detonates, the damage will be less severe."
If the gem is under the chest plate I assume its not visible, so how are the players suppose to know it will blow up? Even if they could see the gem they probably wouldn't know what it was for since most golems have gems in their chest that keeps them alive.
Actually they COULD see it. The chest plate wasn't a tight fit with the other plates. The design of the automaton was mechanical, but it had large gaps between plates and its construction was such that you could see the internals a bit. I showed them a picture of it during gaming, but since I didn't create the picture (I found it on the web) I don't want to post it in this forum. But anyway, they were able to see the gem.
It's been a long time since I wrote those notes, but I'm pretty sure there is a lot of text which is actually DM to reader from an omniscient point of view. The players didn't really know that the gem was going to explode. But I think that they kind of guessed something bad was going to happen based on several clues - 1. an explosion on one of the other ships, 2. I think they had had a previous run in with an automaton which had a self destruct mode, 3. I think prior to self destruct the gem started to power up with visible and audible changes...
Besides, I didn't want to blow them all up with a fireball at such close range. I gave them a subtle hint or two and they figured it out that they needed to get this thing off the ship as fast as possible.
ok thanks for the extra details.
btw when are you going to put the end of the campaign up?