In G Canon, the only journey into the Dark Side Cave we see is Luke. He goes in, bringing his lightsaber.
Where, as Wookieepedia describes (emphasis mine),
Luke confronted an illusion of Darth Vader within and defeated it in a lightsaber duel, dipping into the passion of the dark side in the process.
Through the entry for the cave, C-Canon level descriptions and references to visions had by Luke, Yoda and one of the Starkiller clones while in the cave. There is also mention of the Starkiller clone battling other clones and exploring the cave.
He faced down other clones of Starkiller and had to carefully traverse some deep pits.
Again, in C Canon, Luke goes back to the cave later.
Five years after Yoda's death, Luke Skywalker returned to the cave. This time, he had a vision of a possible past, one where Mara Jade accompanied Jabba the Hutt's retinue to the Sarlacc. She was able to pull Luke's lightsaber away from him, and Luke was pushed towards the Sarlacc when his vision suddenly ends.
On this visit, he has a vision where he almost dies, but the vision ends before he does.
How "real" were the battles that Luke and the Starkiller clone? Were they completely in their head? Were Luke and the clone actually swinging their blades, battling visions that weren't really there? Could someone actually die in the cave due to what happens in their visions?