Hey guys, I've been running a campaign for my group for about a year now and we are reaching the end of the journey. I've incorporated items and themes and cults the feature the Nine Hell's and Tiamat. I want to give my players a real challenge, as they are leveling up in two sessions to level 20, and I want to have their final battle be against Tiamat herself. They have slayed two dragons (a white and a blue) and I feel that in order to give them fulfillment I can't just keep teasing them with Tiamat.

My party consists of a War caster Cleric, a Tabaxi Bloodhunter, a Fire Genasi paladin, a Goliath Fighter, an Aarokrocra monk, a Human Rouge, and a Woodelve Ranger.

Any tips from you all about how exactly to run a battle with Tiamat? I have a setting, but in looking over Tiamat's stats this will defiantly be interesting. I know that a TPK is possible, and my players have come near them before but they also know how to work together and come through when it counts.
 If you guys have any tips on how to GM/DM this, let me know!

Thanks all!

As a boss, I'd have to do some number crunching for how difficult I'd expect Tiamat to be (7 level 20s is a lot of damage and hp), but her basic mechanics aren't terribly interesting, just high numbers (typical optimal tactics are, every round, claw/claw/tail/LA bite/LA breathe/LA breathe, possibly using Divine Word to finish off people who are at low hp or dying), so you might want to give her minions, lair actions, or other special mechanics to spice up the fight.

Don't pull punches. 7 level 20s, especially fully equipped with magic items, should be able to bring some serious pain, so don't let Tiamat be a pushover. It's the epic final battle, so it's okay to kill one or more PCs (hitting downed characters before they can heal is exactly the sort of smart, evil thing she might do). Good luck!

For example, a way to spice up Tiamat slightly:

Trait: Head Loss. At 500, 375, 250, and 125 hp Tiamat loses a head (attacker's choice). When this happens, Tiamat loses a legendary action, and spawn a Young Dragon of the same color. Its initiative is immediately after the turn on which the head was lost.

Absorb Minion. If Tiamat is missing a head, she may attempt to absorb a dragon of the same color instead of a claw attack. The dragon must succeed on a DC 26 Fortitude save or be absorbed (dragons are selfish creatures; a legendary dragon will probably use legendary resistance if it can). On success, she gains temp hp equal to the HP of that dragon, grows a new head of the same color, and regains the legendary action associated with that head. The head is lost when the temp HP are lost, and it uses the damage of the absorbed dragon rather than Tiamat's normal damage.

Getting to the encounter in the first place should be a challenge. She has mates and religious fanatics. The cave or temple should be adapted to creatures that can fly and/or swim.

You could take a hint from Neverwinter and have her attack from portals. Then the portals can close and open someplace else.

Have you got access to Rise of Tiamat?  The final battle is quite interesting in the way it includes lots of factions and minions.  Okay so the factions part might not be worth bringing in but you might gain some inspiration on the way minions can be used and how fighting/overcoming them might help weaken her in one way or another.  Sorry if this is a bit vague but I think I'd start there.

Cheers
Blakey

I have, but that is good advice, I'll go read up on it again and see what I can come up with. Thanks!

Ooh, I like this. This would work really well and would defiantly keep combat interesting for the duration.

While you don't want to take it too far (they're very different mediums), there is a fair amount to learn from video game bosses. Some key general features:

• Give people multiple things to worry about. Focus fire on one target is boring.
• Force people to move. Boss movement, secondary targets in varying locations, and damaging zones are good at this.
• Change it up. Introduce new things to worry about (and possibly expire old problems, or count on them expiring because the PCs kill them) so required tactics change from round to round.

GIVE HER MINIONS! With 7 level twenty characters, she is going to survive less than 3 rounds on her own. Make sure to use her lair to her advantage and consider adding an ancient dragon or a couple adult dragons, or maybe a handful of young dragons to protect her (without their legendary actions) .  I also suggest fudging her hp so she dies at the most dramatic possible moment but that's just me.

Quote from MiroRibeira >>

GIVE HER MINIONS! With 7 level twenty characters, she is going to survive less than 3 rounds on her own. Make sure to use her lair to her advantage and consider adding an ancient dragon or a couple adult dragons, or maybe a handful of young dragons to protect her (without their legendary actions) .  I also suggest fudging her hp so she dies at the most dramatic possible moment but that's just me.

 Why the strike through? 

D&D is all about drama. Sure you could have boss die on the first round without doing anything if the party lucks out. But that is BORING.