Hiya!
I hate to say it, but there isn't a "tanking" capability in 5e. In MMO's, where the real 'concept' of tanking really originated, can have a "tank" because of the mechanics of the computer programming. It's based on "x damage, y defense, z proximity"; all purely numbers based. Intelligent decisions for monsters aren't made...they are simply evaluations of these three (or more) factors, the numbers crunched, and the creatures target selected. There is simply
no such thing in an actual table-top RPG like 5e. You can have a uber-tough (high AC, high HP, high Damage) PC and it will not make a lick of difference in a lot of fights (at least as far as "aggro"...drawing off attacks from others) because monsters react in a more "human" way becuase the DM is playing them as he would expect them to act. No computer script compairing numbers to decide. TTRPG's don't generally work that way.
That said...if you are interested in "protecting others", you're probably good choosing a Paladin or a defensive styled Battlemaster Fighter. As someone mentioned above, a defensive Battlemaster with a shield has the best "defend others" type of abilities. That said, there is still NOTHING stopping the troll from trying to eat the wizards head, even if you are standing next to it. So...you would simply be another target for it to consider, as opposed to you being able to somehow "tank" the troll into attacking you over the wizard.
If I was you, I'd go with Paladin. I mean, fighting demons in the underdark? How can you
not want to play a paladin?!

Sword, shield, the whole "shining knight" thing. Mmmmm.... Make yourself a desirable target to demons; I mean, any demon worth his salt would
love to sink his claws/teeth/whatever into a paladin over pretty much anything else other than a pious cleric...maybe. A DM looking at the situation, say a Defensive Battlemaster or the Wizard ... or ... a Paladin or the Wizard? No contest. Paladin, every time.
^_^
Paul L. Ming