I am planning an encounter at a ancient blue dragon's lair against my pcs.  I am planning on using the burrowing movement in the fight more than the flying.  I can have the dragon burrow to different parts of the battlefield under total cover and then emerge to blast lightning.  Then, burrow again until the blast recharges or I can isolate a pc for claw and bite attacks.  I think the fight would be tougher and force the players to plan more to avoid death from below.  Sound good? Still will use fly speed as well.  If I stay on the ground surface or small heights with flying I think the fight could be over too fast.  The paladin and rogue are packing some heavy damage (great weapon and sharpshooter).

Also, I want to homebrew an attack which is the dragon pulling someone underground.  I am looking for ideas on what that would look like.  

The main strategy seems absolutely spot on for a dragon that can burrow...what the players strategy would then likely look like is to ready attacks for when the dragon breaches the surface, so as long as they are thinking it should still be a fair fight.

Regarding the homebrew...the rules-abiding way would be for the dragon to surface, use its action to attempt a grapple (using the attack action), and then if successful drag the creature as it burrows back down.  This could be devastating to players, as they 1) may not have a means to move through the earth, 2) possibly be restrained, and 3) would not be able to breathe and may start suffocating.  I would greatly caution against using a homebrew means of doing this other than this standard method.  There is a lair action that allows for a similar effects anyway.

I wanted a truly terrifying attack beyond the breath weapon. Dragging the target down felt pretty scary. There is always grapple, fly and drop, but they have feather fall, winged boots, etc. (I will still use the grapple and drop move, but it will mainly be a resource loss maneuver for those forced to spend a spell slot on feather fall rather than a damaging one) 

I think the biggest thing with that kind of attack is specifying how a creature escapes from the earth once pulled down.  Unfortunately not a lot of characters have access to means to move through earth, so without a planned escape maneuver such a move would effectively remove them from the fight and likely be a death sentence.  Also, even with its blindsight, the dragon would have to pierce cover (ie, erupt from the ground) to target a creature, which would still open it to attacks via reactions.  That said, maybe something like this?

Burrowing Snatch.  While burrowed, the blue dragon erupts from the ground around a creature it can see, attempting to bury it.  The targeted creature must succeed on a DC 23 Dexterity saving throw or be knocked prone, pulled into the earth and buried.  A buried creature has full cover from attacks and effects above ground, is restrained, unable to breathe or stand up, and takes 10d8 bludgeoning damage at the start of each of its turns.  While buried, the creature or an ally within five feet of the buried creatures space can use its action to make a DC 23 Strength (Athletics) check, ending the buried state on a success.

The damage per turn is comparable to the ancient blue dragon multiattack, and the saves are the same as its breath weapon.  if you are using a smaller dragon, you will need to reduce the damage and lower the DCs

I like it  

Maybe the recovery Strength (Athletics) save is a tad lower as you are dealing with the earth rather than the dragon itself. Maybe a DC 20? 

DC 20 seems fine for the strength check

Also, in case it wasn't clear, this would be one of the dragon's actions

An ancient dragon might be big enough for a swallow like attack like a purple worm. There is actuality a lightning dragon in radiant citadel which has a similar move. Otherwise if you simply want a grab and drag then just add a grappled to it's attacks.  Movement is separate to attacks so you can combine it with that in different ways to make breaching, diving and abducting attacks. 

If you want something more epic I have some suggestions:

• An interesting idea would be to give the dragon the ability to teleport as a bolt of lightning. Give it lightning bolt,  chain lightning  or use it's breath weapon and have it aim it at the ground or the clouds and let it teleport up and down between the two using it replicating actual lightning.
• A variation of lightning lure that it uses to group up enemies. That's something I might use as a passive aura effect actually for example if you end your turn within 15 ft of another creature you have to save or the spell pulls you together.  It may be an additional save that happens for any one within a certain radius of a pending breath attack so they need to save or be pulled into the path.
• If you want to focus on the breath weapon keep in mind the blue dragon has a line breath which typically struggles to hit multiple targets making it one of the weaker breath weapons. You could have it do a chaining breath like chain lightning, an explosive breath like fireball that explodes at the end or maybe it just does a ball, switch it to a cone or weaken it and make it more frequent. 

Most blue dragons prefer ranged combat, but if it burrows up and isn't within 70ish feet of any PC, that could work. Otherwise, it can keep burrowing around. As for pulling a PC underground, that could work. The creature could maybe be blinded, have half-cover, and be unable to breathe, plus maybe some damage if appropriate.

Quote from iconarising >>

I think the biggest thing with that kind of attack is specifying how a creature escapes from the earth once pulled down.  Unfortunately not a lot of characters have access to means to move through earth, so without a planned escape maneuver such a move would effectively remove them from the fight and likely be a death sentence.  Also, even with its blindsight, the dragon would have to pierce cover (ie, erupt from the ground) to target a creature, which would still open it to attacks via reactions.  That said, maybe something like this?

Burrowing Snatch.  While burrowed, the blue dragon erupts from the ground around a creature it can see, attempting to bury it.  The targeted creature must succeed on a DC 23 Dexterity saving throw or be knocked prone, pulled into the earth and buried.  A buried creature has full cover from attacks and effects above ground, is restrained, unable to breathe or stand up, and takes 10d8 bludgeoning damage at the start of each of its turns.  While buried, the creature or an ally within five feet of the buried creatures space can use its action to make a DC 23 Strength (Athletics) check, ending the buried state on a success.

The damage per turn is comparable to the ancient blue dragon multiattack, and the saves are the same as its breath weapon.  if you are using a smaller dragon, you will need to reduce the damage and lower the DCs

 
This is a good option. One nitpick: you don't need to specify that they can't stand up when they're restrained, because their speed becomes 0. Also, to deal with the lack of sight, give the dragon tremorsense.

Hm. 

For one thing, I feel this encounter needs to be outside, in a ruined temple half-buried in sand. For another, in the name of fairness, there needs to be like ancient walls or other stonework the players can retreat to, to avoid the getting-buried alive attack. 

Otherwise, this is a #PC's x turns TPK. There's literally no reason for the dragon to fight, it can just go PC to PC, bury them all, then return to it's hammock and take a nice, relaxing nap while they suffocate with no recourse. 

I did notice the strength check to escape, but ... why wouldn't the dragon leave each PC 20' below the surface? Realistically, no one would be able to free themselves. Also, while the barbarian can make the check with relative confidence, the wizard cannot. And dragons are clever enough to leave the really physical guy for last - then just roast him with breath weapons instead. 

Quote from Acromos >>

Hm. 

For one thing, I feel this encounter needs to be outside, in a ruined temple half-buried in sand. For another, in the name of fairness, there needs to be like ancient walls or other stonework the players can retreat to, to avoid the getting-buried alive attack. 

Otherwise, this is a #PC's x turns TPK. There's literally no reason for the dragon to fight, it can just go PC to PC, bury them all, then return to it's hammock and take a nice, relaxing nap while they suffocate with no recourse. 

I did notice the strength check to escape, but ... why wouldn't the dragon leave each PC 20' below the surface? Realistically, no one would be able to free themselves. Also, while the barbarian can make the check with relative confidence, the wizard cannot. And dragons are clever enough to leave the really physical guy for last - then just roast him with breath weapons instead. 

 
Good point. This can be partially addressed by making the burrow thing a recharge ability, so the dragon can't just spam it. And yes, put isolated tiles of stone to give the PCs a haven, but make them disadvantageous to camp on.

The thing about monster tactics is that there needs to be a balance between behaving intelligently and utilizing its abilities to its advantage and presenting a boring and frustrating encounter for the party.  This sounds like it's going fully into the latter, because it gives the dragon a perfect defense where it can simply pop underground and be immune to everything the party might want to try and never has to commit to engaging them in a way that gives them the ability to retaliate.