The Aarakocra informed them that basically they could go either left or right of the river dividing the valley. To the left was gnoll territory; not the most fearsome of adversaries, but many of them. To the right was the lair of a manticore, which was a much more dangerous beast, but alone. They went for the manticore. Now 6 adventurers hitting a single monster can make for short and boring fights. To avoid that I had placed the manticore out in the open, in plain view, so the fight started with the monster and the group 200 feet apart. Which just happened to be the maximum range of the manticore’s spike-slinging tail attack. 🙂 This ended up being a rather interesting fight, with the group using ranged attacks while closing in, and being under fire themselves. They won, but two group members went down during the fight and had to be saved by their team mates. In most rounds I decided to let the manticore fire its spikes on the closest enemy. However at one point the closest enemy was the druid who just had changed into bear form. So at that point I let the manticore make an intelligence check, which it failed, leading it to believe that this was an actual bear who had somehow wandered in and would be scared off easily by a threatening roar. At least the druid liked that idea; the halfling monk who got shot instead maybe not so much.
That fight got the group to level 4, so they took a long rest and leveled up. Then they continued up the valley, to the point where the stream was easy to cross. There the stream came out of a cave, and the Aarakocra had told them that air cultists had been seen hanging around that area. So they explored the cave and found three air cultist spellcasters praying in front of a great door. As the cave was only as broad as the stream, the air cultists did get good use out of otherwise rather useless gust of wind spells (I ruled that a gust of wind gives disadvantage to non-magical missiles like arrows, because I was so surprised that in rules as written the spell doesn’t even do that). But they weren’t very strong and the party easily beat them.
The portal was showing a symbol they knew from the book on fighting elemental evil: It was the runes of the 4 elemental cults posed around the elder elemental eye. The door was clearly magical, and it appeared as if one needed the 4 runes of the 4 elemental cults to open the door. As they didn’t have any yet, they went on towards Feathergale Spire. They found some corpses at the base which suggested that sometimes people were thrown off the spire. At this point we ended the session.
[DM’s note: The magical portal isn’t in the Princes of the Apocalypse book. In the book as written you can go right down into the underground elemental dungeons. Only that of course the first dungeon downstairs is level 7, and would be quite deadly for a group of level 3 or 4 characters. And there would be no reason to visit the above-ground 4 elemental keeps, thus skipping a lot of the content of the adventure. A magical door on every entrance, with the 4 keys being held in the 4 keeps, seemed the logical solution for this design flaw to me.]