non player hero

the goblin king loves to hoard his loot but also knows the benifit of bargaining and will offer occational specials of research, troops and more.

basic ideas:
the goblin king has gain his loot with the help of his elite troops, aka. max lvl with passive ability, and is willing to loan them for a handsome price

for gems he will give you a goblin worker for a week which works twice as fast as standard workers

for gold the king will train your troops special tecniques known to few
eg. tome of the dead (witch) chance skeletons will spawn with a bow and arrow