Goodness, gods freighten me. Specifically because there is no indication to how powerful they could be in physical form.
There exists the god Tsarnok- god of magma, fire, evil, and destruction. Thrice have his clerics come to your fortress, thrice you have turned them back with mockery and scorn. Thrice he has sent fiery abominations to press his will, thrice your warriors have turned them back through force of arms. But that is about to change.
He stands before your fortress and calls out
'Hear me, maggots of the earth. I gaze upon you and your paltry holes in the ground and I find you lacking. Bring forth tribute worthy of me or else face my most righteous wrath!' Of course, being a bunch of idiots unworthy of life, all of your marksdwarves open fire... only to have their wooden arrows incinerate before they ever reach Tsarnok.
'So be it. It seems I must teach these maggots their place. Behold!' And, with that, the god opens his mouth and vomits forth a continuous stream of magma that spills in through the fortifications and burns said marksdwarves to little more than fumes and trace elements in the slowly congealing obsidian. Imps, magma men, and a named Spirit of Fire rise from this growing pool of fiery death.
Melee warriors unthinkingly rush out and perhaps a few strike down some of the god's lesser minions but soon they too fall to the heat and the lashing whip of Tsarnok himself. Spatters of molten steel from the armor of the dispatched splashes the sides of your fortress and harden there as eternal testament to one fortress's folly. When no more are to be had, when even the pettiest of wrestlers has been rent into sizzling fat, the god steps forward and sunders the adamantine gate from its moorings with an offhand strike. Then again he steps forward, melts the raised drawbridge from across an empty channel, and bridges the gap with a span of obsidian still shimmering with heat.
'Your finest warriors are mine, maggots. I have accepted them as sacrifice. Now I demand of you this: a shrine to me be built with a ceremonial fire of burning wood kept lit eternal by no less than three clerics. I demand of you this: twice a year you shall sacrifice one of your number to me in this sacred flame. I demand of you this: a moat of magma one hundred dwarf-lengths long kept filled at all times. Do these things and you shall live and perhaps one day find my blessing.'And with that he reached forth and touched the dwarven king upon the chest, leaving a burning handprint that would never fade his flesh.
'Remember this, maggots.'And thus did begin the Years of Fire under the Burning King. United by terror and, later, the promise of power the kingdom united a never before. Their army rebuilt they swept across the earth to forge great temple-ziggrats to their god and master. Untold masses of elves, humans, goblins, heretic dwarves, and far more exotic victims were cast into the sacrificial flame. And thus did the world burn.