ok, so on the lower right path of the third room, where you have to pick a path and find Lilith, there's a tentacle stone that blocks you path to a room with a yellow egg in front of the door, how do i get by this stone? do I need the Tentacle sword?

To my knowledge, you don't.  It's a dead-end path meant to get you the recipe, followed by you jumping down the hole to eventually get back to the first room of the dungeon.  This held true even on revisiting the dungeon once cleared with Leah having the tentacle sword equipped.  I think the blocked-off room is just meant to make the terrain look interesting.