This has to be the most badly worded spell I've seen in a long time. After reading it three or four times, this is how I think it's supposed to work.
(assume CL 15)
Round 1: Take 15d4 Acid damage, then make a Ref save. If you save, you still take 15d4, but don't take any continuing damage.
Round 2: If you failed the Round 1 save, take 6d4 save-for-half.
Round 3: If you failed both the Round 1 and Round 2 saves, take 3d4 save for half.
Any other interpretations?
(assume CL 15)
Round 1: Take 15d4 Acid damage, then make a Ref save. If you save, you still take 15d4, but don't take any continuing damage.
Round 2: If you failed the Round 1 save, take 6d4 save-for-half.
Round 3: If you failed both the Round 1 and Round 2 saves, take 3d4 save for half.
Any other interpretations?