So they fought their way up to the next level. However that level was a trap: The floor appeared solid, but the middle was weakened and ready to break. With the druid in bear form and the cleric both standing on that part on the floor, it broke after 3 rounds of combat. That landed the druid and the cleric downstairs in the burning rubbish, where 5 magmins attacked them. Meanwhile the others had killed all the guards and progressed to the top floor. There Elizar, the keep leader, had cast a wall of fire across the room, which gave him time for an incantation to wake up the fire elemental outside the tower in the burning wicker giant. The elemental then came into the tower at the bottom, where the druid and cleric were. Between the boss and the elemental and already weakened from the fight against the guards, this was a rather tough fight. But the group won, looted the boss, and escaped the way they came before the rest of the camp arrived.
As this had been their strategy in the previous towers, kill the boss, grab his key, and escape, they never got the chance to interrogate any cultists on what all this was about. That was a bit of a problem, as they now had the 4 elemental keys to open the magical portals to the temple of elemental evil, but no clue what was in there or even why they should go there. So this time I had put a letter on the boss explaining the situation. Sometimes you just can’t be subtle as a DM. The letter told them that the 4 prophets of elemental evil were in competition, each trying to be the first to summon his prince of elemental evil. For that they needed to sow elemental chaos, which gave them the energy for the summoning. According to the letter the prophetess of elemental air was ahead in that game (I made that up because the air temple is the lowest level one).
Then in another not-so-subtle move I reminded the group that they had a friendly NPC junior cultist in the air keep. So they went there and got more information about the layout of the temple of elemental evil: A square of 4 elemental temples at the first level below ground, then the fane of the eye connecting everything, and then on the lowest level the 4 elemental nodes from which the princes would be summoned. Well, they got the message and now went into the air temple, but not before the group cleric cut the friendly NPCs throat and turned her into a zombie with his animate dead spell. From the entry point there was a long tunnel leading to a big cave with an old dwarven city, with a step pyramid in the middle. That was now the Temple of Howling Hatred.
While the sorceress didn’t have much opportunity to use her new fireball spell against the fire-resistant or immune fire cultists previously, she now could use that spell. So the first three rooms went: Door open, fireball, cultists mostly dead, wipe up the rest. As the third of the rooms was smaller than the other two, I used an old rule which I had in mind from previous editions, that the fireball had a fixed volume and would blow beyond his 20 feet radius sphere when cast into a too small room. That burned two of the group members. My bad, that rule doesn’t exist any more in 5th edition, so I had to apologize later and say that in future that wouldn’t happen any more. Oh, and lightning bolts don’t bounce of wall any more either.
Reaching the pyramid they noticed a mage on a wyvern on top, who promptly attacked them. That ended up being the second ultra-hard fight of the session. The mage used a lightning bolt to good effect, then turned invisible, and then used another lightning bolt on several group members. They managed to kill both the mage and the wyvern but then retreated due to being heavily wounded and out of resources. At that point we ended the session.
We will play more sessions, but probably not until the end of the campaign book. I told my players that I would like to stop being the DM later this year, and that somebody else should take over. I had somewhat run out of enthusiasm for this campaign, and after over 6 years as DM of that group I think I deserve a break. The next DM will take some months to prepare his campaign, and he is planning to let everybody keep their characters. So I’ll make a character of their level and join the group.