Hey guys, does anyone have any suggestions for monsters with interesting abilities or reactions that will make the players think tactically in a combat? I'm using any of the Wizards books or anything from Kobold press (my rule for the group is if it's in a book I own then it's available to the campaign).

For example, in the Abbey quest from Ghosts of Saltmarsh, there is a glass statue that when attacked with a melee weapon, the attacker receives D4 damage as shards of glass break away. The sorrow sworn also have some interesting ones, such as The Angry will gain advantage on attacks if it takes damage, The Lost will deal damage to a grappled creature if it takes damage.

The fights then have an interesting twist where the players need to figure out whats going on and then how to deal with it. Preferably suggestions for aberrations would be preferred, as that fits into the storyline. These can be anything from CR1 through to 14 or so, the party is 5 level 11's, so I can adjust the encounters to suit. I'm not really thinking spells that can be cast or actions the enemies take, mainly things that the players cause by just blindly attacking.

a lot of Mephits explode when killed.

If you're not adverse to homebrew, give a couple of bandits polearm master and sentinel, with glaives, and see how the players like it!

Anything with illusion magic and invisibility is good, as they can appear to be invulnerable.

I once used a giant in my game with a small wooden platform on the back of the shoulders and on it a goblin wizard who was kind of steering the giant and casting spells. I gave the goblin some bonuses on protection (from being hidden from view but can also be magical). That was a lot of fun for me and the players :) But more generally I often go through the monsters in the monster manual until I find something cool and then adjust the monster to fit the environment of the players. It's easy to describe for example a beholder as a powerful sorcerer with a ray shooting staff. That way the book suddenly gives a lot more options :)

I gave one mini boss a shield that reflected failed spell attacks (such as an Eldritch Blast that didn't make the AC) as wild magic. That was quite fun. It needn't be a shield and could be any item, but it just lent some extra flair to the rare moments when a spell attack failed. At the time I was trying to guide a particular player into using other abilities beyond EB, but it turned out that we all had incredible fun with it. At the end the player characters decided to celebrate the defeat of the mini-boss by taking the shield and attacking each other (intentionally missing) just to see what wild magic effects happened. It was pretty fun session.

I also homebrew created a creature called the 'Stacked Construct'. Basically with every 5hp wicked away by an attack a layer of metal would fall (think a russian doll but in construct humanoid form). Each layer would fall like scales revealing a new slightly lower AC along with different vulnerabilities and resistances and the individual layers could be used in the forging of magic weapons (+1,+2,+3 weapons and ammo) or a magic armour (again +1, +2, +3). The party in question had nagged for ages to be able to craft their own equipment but were insistant they wanted to create something special. So I created the enemy as a way of giving them a fun challenge and also to fulfil the desire for materials from which equipment could be crafted.

Some of the most interesting foes are mages with a penchant for rarely used spells.

guards and wards, forbiddance, Druid’s Grove — look at spells sixth level  and above. There are ideas there that many players would never consider having to face because they think about what they would do with spells, not what someone might do with spells to them.

The most dangerous of the mid level bbegs in my next campaign is a Wizard who hates all adventurers because he isn’t trying to zap them to dust, he wants to break their spirit and drive them to madness (game madness, so literally) and uses illusions and wards and other quirky spells to create hell on earth for them.

hopefully he will get his the second or third time they meet, as the first encounter is around 6th level.

Quote from AEDorsay >>

Some of the most interesting foes are mages with a penchant for rarely used spells.

 
This also goes for spellcasting baddies.

Plant growth is a terrific spell to use against the PCs in a wilderness setting. Give it to an evil druid or a humanoid shaman (or a coven of hags)...especially if the baddies have ranged weapons. A lizardfolk shaman with that spell and a few ranger-type lizardfolks with longbows can be devastating to a party. It comes close to nullifying the effectiveness of front line tanks; they'll take a looong time reaching the enemy who can all the while be firing arrows or ranged spell attacks.

Quote from Xukuri >>

Quote from AEDorsay >>

Some of the most interesting foes are mages with a penchant for rarely used spells.

 
This also goes for spellcasting baddies.

Plant growth is a terrific spell to use against the PCs in a wilderness setting. Give it to an evil druid or a humanoid shaman (or a coven of hags)...especially if the baddies have ranged weapons. A lizardfolk shaman with that spell and a few ranger-type lizardfolks with longbows can be devastating to a party. It comes close to nullifying the effectiveness of front line tanks; they'll take a looong time reaching the enemy who can all the while be firing arrows or ranged spell attacks.

 Imagine if that same group of enemies also had either spike growth or wall of thorns. That'd be a really interesting tactical scenario.

This is not terribly creative, but a coven of Bheur hags on gray staffs is a fearsome enemy.  They can truly pulverize a party with ranged spells (including area of effect spells) if the party is not ready for rapid, aerial combat.  

Quote from Velstitzen >>

This is not terribly creative, but a coven of Bheur hags on gray staffs is a fearsome enemy.  They can truly pulverize a party with ranged spells (including area of effect spells) if the party is not ready for rapid, aerial combat.  

 
As a supporting note, I recommend always customizing the spell list for any coven of hags.