Imagine this:
To tank, you summon a gigantic Felguard which can move around the battlefield at high speed, striking directly at the souls of your enemies and unleashing Felstorms to draw the aggro of every enemy nearby. You see a mob attacking your fellow party member, and send a horde of Imps to harass and distract the mob away from them, kiting them over to your Felguard. As this happens, you see the nearby Hunter accidentally pull another trash pack, leading you to use your cooldown to summon a Demonic Tyrant, instantly healing your Felguard to full and granting it an absorb shield.
You directly take control of the Tyrant, and use its Demonbolts and swiping claws to aggro the entire pack at once (Replacing Shadow Bolt with the claw attack). At the same time, you command your Felguard to Soul Strike over to the pack, bringing them all together and allowing the party to burst everything down.
Alternatively, if your Demonic Tyrant is on cooldown, you can call your Felstalkers to harass and push the enemies towards your Felguard, nipping at their heels with Dreadbites.
Your Felguard’s health is low, and you don’t have Demonic Tyrant up! You have multiple options to bring its health back up- Health Funnel is efficient, but dangerous to you. You could use Fel Domination to resummon it at full health instantly, but it loses all aggro- instead making the mobs it was distracting attack YOU.
Demonology, rather than playing like a normal DPS or tank, feels more like an RTS in WoW. Instead of dealing massive amounts of damage yourself, you conduct a demonic symphony to command the battlefield and lead a one-lock army to victory.
The spec is by far the hardest to play in the game, but as a reward it has the highest potential for damage out of all the tanks- if played optimally, you could potentially reach up to 95% of a DPS! If played poorly, however, you’re the squishiest tank in the game and could simply die as fast as a DPS without a Felguard protecting you.