The wise woman mentioned a "dead tree behind 2 rocks," which could refer to a number of formations in the northeast clearing of Crawky Forest. The specific tree/rock combination you're looking for is on the west side of that clearing. You'll have to hit the "cracked" rock on the right five times, and have all seven Crystals in your inventory, to open the dungeon of Golvellius.
This is a side-scrolling dungeon that uses all of the tricks of previous dungeons. Several times you'll come across a series of platforms where falling down means leaving the dungeon. You'll also fight several Giant Snakes with no terrain protection. After the second, you will have to jump and break several blocks to proceed. The exposed lower path is a dead end.
When you see a lower path with a Red Snake, ignore it. You can make a fair bit of progress by that path, but it eventually leads to a dead end. Likewise, don't fall off the upper path. You'll find a Giant Snake with homing Fire Spirits, but it isn't hard to take out safely. The next Giant Snake has no fancy projectiles, but its position on a set of stairs after a hallway make it tricky to fight. The same is true of the fifth Giant Snake, which fires one Fire Spirit at a time.
Eventually you will find a series of swords and shields. Stand under the two-square platform and hit the set on the right to lower that platform and open the way forward. This is followed by a Giant Snake that spits out truly ridiculous numbers of Red Snakes. That turns out to be the last guardian before Golvellius, so make sure you grab all of the healing tokens.
Golvellius
Golvellius starts out casually strolling around, but will eventually breathe a long line of fire. The fire deals a base of 6 damage, but that's nothing compared to touching Golvellius, which deals 18. Even with Ramurasu's Pendant, you can't take too many of those hits. Golvellius will always follow up his fire attack by chasing after you, swinging his huge arm blades. Therefore, start running when you see him breathe. Don't try to run under him, because the fire will block your path.
Golvellius will occasionally start chasing you without warning, but there isn't much you can do about this. Just keep hitting him while he's meandering around and running away when he starts attacking. The Legendary Sword will take him down before too long. Congratulations, you just completed Golvellius! Enjoy the rather bizarre ending. Sadly, the sequel that was strongly hinted at never got made.
You can "continue" to start a new game, but (aside from a bug where you keep any Rings you still have), there doesn't seem to be anything different about this playthrough.