I'll be playing a D&D 5e Warlock/Paladin character in the Curse of Strahd adventure. I'll start off by taking 2 levels of Paladin (variant human with War Caster) and then start taking levels in Celestial Warlock until I'm Paladin 2/Warlock 9.

My plan is to get the armor proficiency with the paladin and to nab some extra spell slots along with Divine Smite. I'll be using Warlock to gain Agonizing Blast and Grasp of Hadar. I'm choosing Pact of the Chain so I can take the Gift of the Ever Living Ones (eldritch invocation) in order to maximize my own healing with spells and class abilities. (Celestial Warlocks can heal as a bonus action.) I want to use my familiar's Help action to give my one attack (with shadow blade) advantage.

I'll be taking the Sentinel feat so I can tank.

My main method of damage is shadow blade past character level 5.

It will start off as 2d8 + Dex mod at 2nd spell level, and 4d8 + Dex at spell level 5. I'll be casting the green-flame blade cantrip until I gain the Extra Attack from Paladin.

My plan is to be a high-damage tank so that enemies notice me and start targeting me while my allies pick the enemies off.

I will be a Dex-based fighter wearing medium armor and a shield.

Will this work? How do I keep the enemies focused on me so that my allies can do as I intend: pick them off while I keep the enemies busy?

The core idea of this build seems to be: you want to have a high AC and be a tank, but you only need to spend two levels on paladin to pull that off, and you're going to spend the rest of your levels on something with higher utility. I haven't tried this myself, but I've seen others try it and it seems to work pretty well.

The thing where you heal yourself as a warlock seems like a good trick. It's only a tiny bit more healing than you would have gotten from Lay On Hands as a normal paladin, but having it be a bonus action is useful.

Smiting using your warlock spell slots is also a good trick.

You've written that you want to have a high damage output to draw aggro. I don't think this will work as well as you want, because being a multiclass character will drop your damage output significantly. In particular, a single-class paladin would get double-attack at fifth level, and you're not going to get that until much later.

You'll get shadow blade instead, but it's not as good: the damage output is less (2d8+DEX with your build, versus two attacks at 1d8+DEX each for the paladin), you're spending spell slots which could be used for other things (like smites), and every time you get hit, you'll have to make a constitution save or lose your weapon.

(Also, tanks don't hold aggro by having high damage output. Tanks hold aggro by having the Sentinel feat.)

As a side note, it's not surprising that it's hard to optimize both AC and damage simultaneously. If there were a build that gave you high damage and also high AC, everyone would be doing it already.

As Andras notes, your current build requires STR 13 and a high DEX. If you switch to a STR-focused build, then your stat requirements go down and your AC goes up, at the cost of some initiative. I'm not sure what your stat array looks like, but one guess might be that you'd go from 13/15/12/8/10/14 to 15/8/13/10/12/14, and you could put a racial +1 in CON to get bonus hit points.

I don't think Agonizing Blast is a good choice for you. Your eldritch blast spell has a low attack bonus because CHA isn't your primary stat; also, it will have disadvantage any time you use it while in melee combat. Warlock invocations are valuable, and it seems weird to spend one for a minor buff to an ability you'll rarely use.

I'm unsure about Grasp of Hadar. You might be able to use it to pull foes out of the air, or to pull them away from allies they were grappling. Those are good effects.

But the thing is: you're also spending an ASI on this (for War Caster). There are lots of super valuable things you could be doing with an ASI and two warlock invocations. The ASI could give you +1 to attack and damage rolls; the invocations could add non-combat utility to your character with Beast Speech or Mask of Many Faces or Misty Visions, or you could just get the Devil's Sight invocation and use the darkness spell.

Viable, but not good

Abilities

You will be very MAD, needing Str 13 for multiclassing, Dex 16+ for attacking, Con 14+ for survival, and some Cha 14+ for casting.

By going with Str as primary, you can dump Dex completely, gaining +1 AC in the process, and losing some initiative.

Patron

Healing Light is great for two reasons:

1. It is like Healing Word, can be used to bring back unconcious allies without having to spend a whole turn on it


2. It is not a spell like Healing Word, so you can use full action spells in the same round

Neither of which matters for you, you want to use it on yourself, which takes away more than half of its usefulness, and you don't intend to cast spells as actions mostly.

I would recommend Hexblade for damage or Fiend for survivability instead.

Invocations

Pact of the Chain is great for scouting, but Gift of the Ever Living Ones is mediocre at best. Agonizing Blast is also not for you, as you want to stay close. If Eldritch Blast is just a backup option for flying enemies for example, you should not waste your few invocations on it.

If you do not have darkvision, Devil's Sight is great, also Relentless Hex is an easy way to keep up with elusive enemies.

Damage

You want to use Green-Flame Blade until you get Extra Attack from Paladin, but Paladin 2/Warlock 9 does not get Extra Attack. You are much better off with Palading 6/Warlock 5 in terms of sustainable damage, and the great Aura of Protection.

Survivability

If you take the Defense Fighting Style, wear the heaviest armor available, and plan on using a shield, you do not have to worry much about your HP, so Healing Light is not really a must.