A very interesting idea for making it so that there can be some variety with enemies in 5th edition. Since 5th edition removed a lot of spell-like abilities from enemies that typically had them in previous editions, a lot of dangerous enemies became simply menacing but not able to back it up in the way my players have come to expect. With this product, I was able to surprise them with a return to old school days when a sudden spell-caster in the midst made a fight last more than five rounds.
It also allows for a far more ominous villain patron when a shaman suddenly loses favor and explodes in front of the party. The trick is whether or not it destroys their equipment, because then it might make the party break down in tears.