When to do Radahn?

[Morkei]

As the title suggests, does anything lock out after killing Radahn? Like, any quests I need to progress/complete to a certain point before killing him, or bosses that I need to do before killing him (if they are locked out after)? I'm just starting Volcano Manor quests now, and have already gone up the Dectus lift and started clearing the outskirts somewhat.

Side question, I read there are 2 bosses you can do in Redmane Castle, but you can't do them if you killed 2 main bosses or something? I haven't been in Redmane Castle (cleared up to the bridge to it). Is this true, and I missed out on their weapons they drop?

[Fripposo]

after kill radan the path to complete rannis quest gets unlocked

[Vezera]

Redmane Castle starts out as one instance, at the end with a boss fight but unable to proceed to Radahn. When you progress Ranni's questline or reach Atlus Plateau, however, Redmane Castle switches out to another instance where most of the area is cleared out and blocked off and Radahn is reachable.

When you kill Radahn and exhaust dialogues of NPCs in Redmane, the instance will swap back into the original upon your next visit.