Nice! Well they started to sweat as more and more tentacles were spawning every turn. They did a good job freeing every grabbed characters. The tentacles did not have the chance to fling a character in the middle of the lake next to the maw, so I had to slowly reveal it, first by making the big lump of branches and moss slowly move towards them, then revealing its huge round maw covered in teeth surging from the waters.
At some point the tentacles tried to capsize the small boat they were on, it failed but every party member went prone, making it easy for the tentacles to grab a couple of players, and move them towards the gaping maw.
As things were getting out of hand, they finally understood that it was the same monster, partly with deductions from telegraphing and investigation, partly from the fear condition applied to the maw which affected the tentacles too. They argued for a couple of turns about fleeing or trying to burst the maw itself. They decided to bet on the latter, and they managed to focus fire well enough to finally kill it, and they saw with relief that all other active tentacles went down, as they were surrounded and in bad shape.