My group just started a Curse of Strahd campaign.  We've made it through the introduction and hit level 3.  The group lost our barbarian whom was going to provide a nice damage sponge for the group. Originally, I was planning on taking Polearm master for more damage, but I think it would be more beneficial for the group for me to increase my defenses and try to become a tank equivalent for the group.

I'd appreciate feedback on transforming my PC into a tank up to lvl 10.  I do not plan on multi-classing unless role-playing forces me out of warlock.  Specific questions are feat or ASI at level 4 and 8, as well as any input on spell choices and invocations.  Specific questions of concern this week for me are Blur vs Mirror Image.

All the information you could want is below as well as my planned build.

Campaign Information

1. Curse of Strahd
2. Levels:  3 - 10
3. Group Composition
   1. Hexblade / Pact of Blade
   2. Monk (new player)
   3. Sorcerer (brand new player taking role playing as priority for spells, no defensive spells chosen)
   4. Twilight Cleric (playing in backline)

Hexblade / Pact of Blade Build

1. Race:  Variant Human
2. Feat: Warcaster
3. Abilities
   1. Strength: 10
   2. Dexterity: 14
   3. Constitution: 18
   4. Intelligence: 12
   5. Wisdom: 8
   6. Charisma: 18
4.  Starting Equipment
   1. Scale Mail
   2. Shield
   3. Longsword

Feats / ASI

1. Lvl 4
   1. Sentinel (current thought)
   2. Charisma +2
2. Lvl 8
   1. Charisma +2 (current thought)
   2. Pole-arm Master (sentinel synergy.  Loose 2 AC.  Lose +1 to ASI)
   3. Other?

Invocation Plan

1. Level 3
   1. Improved Pact Weapon (until a magic weapon is available)
   2. Mask of Many Faces (out of combat utility)
2. Level 5: Thirsting Blade (more attacks)
3. Level 7: Tomb of Levistus (lose a round, but don't die)
4. Level 9: Relentless Hex (mobility with hexes)

Spell Plan

1. Level 1
   
   1. Armor of Agathys (great scaling, temp HP and dmg in one spell)
   2. Hex
2. Level 2
   1. Blur or Mirror Image
      1. Blur takes concentration, but lasts through more attacks.  I do have +4 Con with Warcaster meaning only 6% chance to lose concentration
      2. Mirror Image.  Takes no concentration, but can fade quickly once images are gone.  I plan on trying to be the focus of attention, which I think makes Blur the better spell at this level.
      3. Swap out for Mirror Image at level 5 to allow for an 8 hr hex concentration
   2. Hold Person (crowd control, scales nicely) 
3. Level 3
   1. Hunger of Hadaar (more thematic than Hypnotic Pattern)
   2. Counterspell (doubtful the sorcerer will take this)
4. Level 4
   1. Shadow of Moil (Blur replacement with thorn damage when I'm hit)
   2. Banishment (nice scaling)
5. Level 5
   1. Synaptic Static (AOE debuff and dmg with no concentration on an intelligence check)
   2. Cone of Cold
   3. Hold Monster? (Save or Suck on one target makes me hesitant)

A thought on feats / ASI - I did this with my melee Hexblade. I took the +2 to CHA at level 4, then the Inspiring Leader at level 8. With that, you can give out 13 temp HP to each character every short rest...

Temp HP doesn't stack, so you'd have to burn thru those before Armor of Agathys, but it helps the whole party.

Solid Invocations. and spell selections. I've found that I rarely use Hex, and have dropped it from my spell list. I'd rather use Hexblade's Curse, which doesn't use up a spell slot, and save my (two) slots for a Smite, or an AoE like Hunger of Hadar, Synaptic Static, Shatter, Cone of Cold...
Counterspell is good, (even if the Sorc takes it... sometimes you really, REALLY don't want an enemy to cast a fireball at your party...) and Banishment can even get you or someone else WAY out of the melee.

Find / buy / kill for a Ring of Spell  Storing. An RoSS is a great way around that 2 spell slot limit. Maybe even cast Hex into it, so it doesn't burn a slot...

Thanks for the feedback Berzerker.

Inspiring Leader:  Inspiring Leader is an excellent idea.  Extra HP to the entire party every short rest would be great.  The choice between "sentinel" and "inspiring leader" is tough.  Both feats provide party support.  The extra hit points per short rest is probably the optimal choice.  I think I will stick with "sentinel" purely because that feat is an active ability rather than a passive one.  Game play wise I think "sentinel" will be more fun.  With "Sentinel" I can also move up to block the way first turn, then cast "Armor of Agathys" or "Blur" and fulfill my role of damage sponge / light control.

Hex:  Even as early as level 2, I too have found I rarely use hex.  Casting "Armor of Agathys" twice was simply better at level 1 and 2.  Agathys "healed" 5 hp and did 5-10 damage per cast.  "Hex" might do 15 dmg if I was lucky.  I cast the spell once, then haven't cast it since.  That may change at level 5 when I can cast Hex and concentrate on it for 8 hours.  I could see using "Hex" and "Armor of Agathys."  After a short rest I will still have "Hex" up and can cast "Agathys" again followed by a "Counter-spell" or another "Agathys."  This would mean no further concentration spells though like "Blur."  Given my role, "Blur would still probably be the better choice.

Magic Items:  Magic items are out of my control.  If I can get a "Ring of Spell Storing" that would be awesome.  The only items of note we've seen so far are a silvered short sword (gave to monk of course) and an unknown cloak.