Every time you enter you must speak with Amulius Ramio and go through the dialogue options until you say "Alright, give me the key I'm ready". Don't just spam click through the dialogue since you must end the conversation with the above option. If you end the conversation on any other option then you won't have 'Maulic's Stronghold' appear as an option when clicking on the gate to Bloodskull Valley.
You must speak with him again if you die or zone out for any reason.
The second area is the field on the other side of The Battered Wall and up to the arena/buildings. Here you will have to fight many trash mobs and two Bloodskull spotters (worth AA) that are found on top of the watchtowers. As long as the spotters are up, two Bloodskull reinforcements will run from the narrow path in the east of the zone to the center of the zone. If they get there, four Bloodskull bouncers will spawn and aggro. Thus, it is crucial to kill the spotters asap to stop this process (although one more bouncer mob usually spawns after each spotter is killed). Once all the spawning mobs are down, and the mobs everywhere except The Bloodskull Arena are down, head to the arena for a ring event that begins the next stage.