SOURCE: Player’s Handbook 

Rating the Benefits of War Caster

War Caster has an array of benefits, some decent, and some truly amazing:

Benefit #1 – 

Advantage on constitution saves to maintain concentration caused by damage

Whoa! Losing concentration is one of the most dangerous things that can happen to a spellcaster; personally, sometimes I’d rather die than have a spell stop working because at least I could be revived later if my spell saved the party. This prevents that worst case scenario quite well.

Benefit #2 – 

Can do somatic components even with a weapon or shield in one or both hands 

This is most useful on characters who mix weapon use with spellcasting, but vital if you want to use a shield or rapier, and cast a spell. 

Benefit #3 – 

Can replace an opportunity attack triggered by movement with a spell that has a casting time of one action and targets only that creature

This is most useful on high AC characters since they tend to make more opportunity attacks, but this is potentially more action economy for your spellcasting.

Mechanics and Requirements

Understanding How It Functions

Advantage on concentration saves caused by damage is fairly simple, but the next two parts of the feat are more complicated.

War Caster only lets you perform somatic components when your hands are occupied, and only if at least one of the things occupying them is a weapon or shield. So, you can’t use this feat to help cast spells with a material component; you will need to access that ability some other way. Additionally, remember, that objects other than weapons and shields do not count, so you can’t cast spells with somatic components while carrying a box in two hands.

Finally, in regards to the second bullet point, there are some ambiguities and odd tricks to consider before taking it; you may be able to drop your weapon, cast a spell, and pick it up again if your DM agrees that it takes no action to pick up weapons (Jeremy Crawford rules people can drop weapons for free, and implies but doesn’t outright state the rules support this, but stresses that this is just how he runs it at his table). I don’t recommend actually trying this though, since the mental image is silly.

It’s also unclear if you can take a hand off a two-handed weapon to cast a spell, but since you just need to use two hands to attack with the weapon, not to hold it, this would make some sense. It certainly makes more sense than dropping your weapon on the floor and picking it up every six seconds.

The final part of this feat that needs exploring is the feat’s third feature and its interaction with Polearm Master: the intent is clearly that the opportunity attack from Polearm Master is made with the polearm itself, but the text just states that you get an opportunity attack, so talk to your DM about whether or not you can replace this special opportunity attack with a spell.

Key Stats

Advantage on concentration saves is good, but you should have a decent constitution anyway. If you cast spells, have a good spellcasting ability modifier, and if you use weapons, have a good ability score associated with your weapon(s) of choice.

Ideal Characters for War Caster

Top Classes

Arcana Cleric – With their access to the Booming Blade spell, shield proficiency, and potent concentration options like Spirit Guardians, the Arcana Domain Cleric can make use of all of the features of this feat very effectively.

Shepherd Druid – For those who want to go nuts and break the game at a high powered table using the Conjure Animals spell, which I don’t necessarily recommend but have to point out the strength of, maintaining concentration is so important that advantage on the saving throw is worth this feat even if the other benefits aren’t so good.

Sorcerer – You’re already proficient in constitution saving throws, you might as well make your concentration unbreakable. Your Polymorph spell will never be stopped prematurely unless you have to make a DC 30 save or something else crazy, and Absorb Elements and Shield should help prevent making those in the first place.

Multiclassing Considerations

If you’re multiclassing for armor proficiency and your dip gives you proficiency in constitution saving throws, ask yourself if you want to turn an almost guaranteed saving throw into a virtually guaranteed saving throw or live a little on the edge and be fine with just a “good” concentration save bonus. Your preferences will determine if taking this feat is a good idea.

Race or Subrace Choices

No particular races synergize well with this feat, except maybe the Earth Genasi if you wanted to concentrate on Pass Without Trace between multiple fights to ambush multiple groups of enemies.

Combos, Tactics, and Synergies

Constitution save proficiency, advantage from this feat, and a high con score will ensure that your concentration never goes down. I once went a whole campaign with this combo while never having my constitution save tested.

Complementary Feats

Resilient (Constitution) both synergizes with and competes with this feat. I prefer Resilient (Constitution) on characters with an odd con score, but it’s personal preference. Polearm Master is also potentially excellent if your DM allows the interaction.

Spells that Synergize

Any spell that uses concentration benefits from this, but only some spells like Eldritch Blast and Hold Person are usable with the opportunity attack part. Booming Blade deserves special mention for being a straight upgrade when used to replace an opportunity attack, and there are some comedic builds possible with Polearm Master, Eldritch Blast, and Echo Knight: the so-called “ghostlance” build may be a ton of fun, but it could be too powerful or your DM may not approve of some of the rules interactions, so think before using it.

Strategies for Maximizing War Caster Effectiveness, Or Just One Strategy

Advantage on concentration saving throws means nothing if you don’t concentrate. Concentrate, wizard, concentrate! Always find an excuse to have a concentration spell up in combat, especially since concentration spells tend to be powerful.

Final Thoughts on War Caster

War Caster begins with an absolute banger of a feature and ends with two solid features: overall, it’s a great feat, both for mixed weapon and magic user characters and straightforward spellcasters, especially heavily armored ones.

It’s one of the strongest feats available and may beat a bonus to your character’s most important ability score, so consider this on any applicable character.