After defeating Nithral if you turn back the way you entered the boss chamber you will find a damaged archway that you can blast through revealing a secret passageway. But for me most of this secret room is blocked off by a rock slide. Witcher sense reveals it to be a large glowing pile blocking your way and it shows small white swirl symbols as if it can be interacted with. The map clearly shows the passage continues past the blockage.

I can't figure out how to destroy or get past this blockage. Tried bombs and every sign I have many times. No joy.

I'm playing on a somewhat easy setting (Story with Challenge). Perhaps only people playing on Hard are permitted to go beyond this blockage to get whatever treasure lies behind it? Or maybe it requires a special type of bomb?

(out of ideas)

That was it. Thanks!