Protection fighting style and sentinel feat, together?

Question

I'm working on making a utility tank that is just no fun to be around. He won't be doing loads of damage but, he'll make it difficult and not very worthwhile to hit anyone but him.

What I'm wondering is, if its worth taking the protection fighting style, and the sentinel feat. Imposing disadvantage on attacks against my allies I'm close to, while also gaining opportunity attacks. This may sound like a dumb question, but do I have to ready both of these individually? And if so, when I get my multiple attacks, will I be able to use them on these skills?

From what I understand, I have to use my full action to ready one of these, but not both. is this correct?

Answer 1

You don't have to use an action to ready either of those. Both of them use your reaction.

Protection Fighting Style:

When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll.

And Sentinel feat:

When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.

In case you don't know what a reaction is, it's this:

Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else’s. The opportunity attack is the most common type of reaction.

When you take a reaction, you can’t take another one until the start of your next turn.

So, in summary, you don't have to use your action to ready either of these. However, since both of them require you to use your reaction, you can't use both of them together. When an enemy attacks one of your allies, you can choose to use either of them, but not both.

Answer 2

Unfortunately, you cannot use them both together

The Protection Fighting Style lets you spend your one Reaction to impose disadvantage, while the Sentinel Feat also lets you spend your one Reaction to make a single melee weapon attack. (Which is not an opportunity attack, unless your DM wants to say it counts.)

There is no readying going on, so on your turn you can still use your Action to do whatever, and maybe move around to position yourself where you want to be.

Answer 3

You don't have to prepare them. But this doesn't work RAW

You can make Opportunity Attacks "for free" as long as your targets move out of your reach. With the Sentinel feat, you can also do this when an ally adjacent to you is attacked, instead of you. It takes your reaction to do so.

The Protection fighting style allows you to impose disadvantage on attacks on your allies. However, it also takes your reaction.

As you get only 1 reaction per round, you can only either make an opportunity attack or impose disadvantage, not both, once per round.