Far too often DM's and GM's short themselves and everyone at the table by failing to use one of the most powerful tools in our arsenal...words. All to many a time descriptions fall on two grim poles of verbiage they are either woefully scant in their brevity or works of mind-numbing encyclopedic prose and accounting of mercurial minutia that beats all fun and excitement out of an encounter just as it starts.
Why not let the villains and fiends our adventurers throw themselves against do the talking?
It's as old as the words of the immortal bard, a corny but evocative tradition of comicbooks, a twisting hint of drama to come.
I'll leave you with three examples of the same situation:
1- A balrog wreathed in flame steps out of the portal.
2- Your attention is drawn to the 20' by 20' metallic portal, it's profane metal runes become stained as by a furnace flame and all hint of moisture is boiled away. A shadowy form is coalescing out of smoke lit by an unseen light from the depths. The now suffering portal formed by the stygian priests of Xelbec who fell before the priest kings of Dorn are now surely open as from that gate now a huge demon wreathed in flames fueled by malice steps out. Its wicked whip some 15' in length formed from succubi hair licks the air like lightning, it's great 10' ebony sword forged by grues in the deepest furnaces of the abyss projects a malice of it's own. The demon grins as you behold the visage of the master of fear and flame.
3- A dark voice booms as a huge demon steps through the dark runed portal with unyielding confidence in it's stride. It speaks:
"Hark,the quivering shadows make haste to flee my presence,
secrets once consigned to darkness are revealed,
coruscating demons of dream ride out on nightmares of eld,
death shall be the last blessing, boiling blood a passing sacrament
a hymnal of screams shall fill the blazing night
your souls are forfeit, before this searing dawn the lucky shall die .
Behold, I am the Master of Fear and Flame."