I’ll keep it short: each of Y’Shaarj’s attacks triggers the passive of Azshara’s Xal’atath (ugh, spelling these is always tricky).
For further context, reminds me also of this one: Yashraaj not affected by freeze - #4 by SparkyElf-2852 , that is, each of the attacks is treated as casting a separate shadow ability (Defiled Attacks is also affected by Cursed Treasure in the same way as described there).