Matthew Deyling is a human mage situated at Lunarfall. It seems he knows a lot about demons for a mage.
"The felguard, front-line forces of the Legion. Almost always seen with heavy, two-handed weaponry. They are slow to strike, but hit quite hard, and are more agile than they look."
"A new breed of felguard, the fel lord towers over his lesser minions and often wields a massive, yet slow weapon. Should you find yourself facing one alone, I would suggest a tactical retreat."
"The pit lord is a massive, terrifying behemoth, don't expect to face one with much less than an army and come out alive. Capable of rending a man in two with a single glaive swing, and burning half of a squad to a crisp with fel energy, they are a threat at any distance."
"The felhound, a quick, wiry beast often seen in large packs. Each hound has two tendrils, ending in a sucker of sorts. Should they latch on to you, they will drain your power and, eventually, your life. Hit them before they hit you."
"The Eredar, one-time Draenei that fell under the control of the demon lord Sargeras. Skilled sorcerers in both fel and shadow magics, adjust your wards accordingly. Often in command of contingents of demon forces, they are both a high threat and an excellent target of opportunity."
"The wrathguard, elite warriors of the Legion's front lines. Often seen wielding two weapons with equal measure, avoid letting them get the upper hand in swordplay, lest you face a rain of blows from which you will not emerge."
"The lowly imp, a creature of felfire and trickery. Not a large threat alone, but often found in large groups which can quickly overwhelm your defenses from a distance."
"The dreaded infernal, siege weapon of the Legion. Expect these brutes to be hammering down your defenses with blazing fists made of fel fire and stone. They will often arrive on the battlefield as meteors launched from afar, then unfurl themselves to cause further devastation."
"The doomguard, commanders of the front-line forces. An excellent target of opportunity that will cripple the enemy's line. Be wary of fire magics and dark curses, as well as their capable strikes in melee."
"We have only recently learned of this variant of the felhound, which we are calling the "felstalker." Faster and more coordinated than their brethren, they are often seen hunting in packs and utilizing a crude form of pack communication. Avoid their bites, as they are laced with all sorts of foul magics."
"The dreadlord, perhaps most insidious of all of the Legion forces. Capable of bending minds to their will, they will turn half of your army against against your before you've even seen one. Take note of their shadowy magics and control over carrion insects, and steel yourself against their frightening illusions."
"We don't yet know much about this new breed of demon, that we have taken to calling the "imp mother." Limited reports indicate that her stomach may operate as a kind of portal from which imps emerge. Use caution, we're not sure what you're up against here."