I need monster ideas.

I'm creating a 2014 5e campaign with 2 fighters 1 rogue and a cleric. they are level 10 right now and I keep throwing level 10-12 monsters at them right now and its not enough. I need something challenging for them. they really like combat. it seems that that's all we do because they like it so much. I would like something that almost kills them and encourages them to be more willing to try some roll play for once. what monsters can I throw at them? 

My BBEG is a level 20 artificer and I hope to get them to level 17-18 by that time but I need something challenging now. 

just for some more back information they are fighting the knights of the purple cross. a group of people that are attacking them because they recognize the BBEG. the BBEG has convinced you that he and his master (there is no master) they are the good guys and that they are using you to get rid of the bad guys. but its up to you to find you that Bill (BBEG) is the bad guy... any way right now they are in a ossuary church and Bill has been fighting with this Good (bad) guy and I just don't know what I can make him./her I'm thinking a cleric but I don't know if that would be too obvious.

what do you guys think? for the next few session they are in the Ossuary and then a library and then they are getting ambushed on the trail to see a town that's being attacked by a war machine. any ideas? any ideas for stats of the war machine?

any help would be great! I appreciate you guys!

There are several stats for huge Colossus, such as the literal Colossus in the 2024 Monster Manual and the Warforged Colossus in Rising from the last War, but those are CR 25, probably would lead to a TPK. A skinned Hellfire Engine from Mordenkainen Presents would be in the CR range you're aiming for and is a fun, dynamic enemy.

It should come at them alone though because it is CR 16

Quote from chapmanmichael42>>

I'm creating a 2014 5e campaign with 2 fighters 1 rogue and a cleric. they are level 10 right now and I keep throwing level 10-12 monsters at them right now and its not enough. I need something challenging for them.

they really like combat. it seems that that's all we do because they like it so much. I would like something that almost kills them and encourages them to be more willing to try some roll play for once. what monsters can I throw at them? 

My BBEG is a level 20 artificer and I hope to get them to level 17-18 by that time but I need something challenging now. 

If that BBEG is built as a level 20 character, the players are going to go through him like a hot knife through butter. Characters are too fragile, and monster stat blocks are better balanced for that. (But an unsupported caster monster will, I repeat, still get smeared without support.) 

Thank you for your advice. If I’m understanding correctly more monsters at once. And my BBEG should have a monster stat block because characters are too easy to go through. 

I give you, for your leisure, a choice. Will you be kind and take the Beholder, or will you decide to end them with the Ancient Dragon?

Awakened Rust Monster Monk.  Yes I know the PC version of the Awakened spell doesn't work but NPCs have different rules.  Watch your Heavy Armor Paladin or Warforged look on in abject fear.

If you are looking at running a combat-oriented campaign but want to encourage roleplay and problem-solving, you need to look not at what monsters to throw at the party, but what challenges and encounters.

As an example, let's say that you're planning on the party fighting a hill giant. Dropping a hill giant into a curiously circular glade whilst boss music sounds is the video-game approach, and will result in "we fought them and then we won/lost" gameplay, which is great for rolling dice and dealing damage, and less great for roleplay or anything other than rolling dice and dealing damage.

As others have said, you can't force the party - if you say "you have to negotiate with a hill giant" they will either kill them anyway, or have their character sit it out until combat, which are both not great to work with.

Instead, think of the conflict you're creating. Why do they fight the hill giant? In this case, this is a "random" encounter, not the culmination of the quest - they are on their way to save the knight from the princess and they find a hill giant.

You need to first introduce conflict - that means more than just fighting. The Giant's goals need to clash with the party's goals. Maybe the giant has the knight's horse, and is planning on eating it. The giant is distracted by preparing its campsite, and the party has an opportunity to sneak in and take the horse back, or to set an ambush, or to just run in screaming, or to intimidate the giant, or to lure it away and steal the horse - there's a lot of things they can do.

Then you can add a complication in the goals. The Giant intends to eat the horse. The party intends to liberate the horse. And the giant's pet Warg also intends to eat the horse, but the Giant is keeping it at bay whenever it attempts to pounce by hissing or shouting at it.

Now the party has a bigger challenge. If they fight the giant, the Warg might get the horse. If they fight the Warg, the giant will be alerted to their presence. Sneaking past the Warg will be a challenge. If they fight both, the Horse may get away, and need to be tracked down.

The goal for you, as a DM, is to present them with consequence. They can just run in and kill, but the Horse might not survive the encounter if they do. If they make a plan - "I can paralyse the warg whilst we kill the giant" - then success - you got them to do some roleplaying!

The most useful thing I ever did in my games was to think about what the monsters are doing. They have goals and plans, and that needs to clash with the party - or align with them, if that's how things lie. The party needs to harvest a fruit, but it's in an ogres garden and they don't like visitors. The Ogre won't be stood still in a loop animation until the party triggers combat - make them feel alive!