She isn't a monster. She is a God.
When the PCs engage her, she is the size of a mountain. She is the terrain upon which the final battle is fought, not the monster they face.
Surrounding her is her brood, chromatic dragons of all shapes and ages. To a tricked out epic party, an ancient dragon is a quick fight. As they battle along the surface of Tiamat's body, they meet ancient red dragons as random encounters.
The surface of her body is a world, and each neck has its own hazards.
In order to have a chance agaist a God, you need a MacGuffin. Tiamat has 5 heads; I'd actually say a 5 piece MacGuffin. Maybe they have to embed a piece of the artifact in all 5 of her heads, battling down each neck, hiding the piece, then going to the next one. Meanwhile, Tiamat is treating them like the fleas they are.
When the 6th artifact is embedded (one for each head, and one for her heart (6 PCs)), only then do they get to fight "Tiamat" as a monster; a fight between them and Tiamat's avatar.
Oh baby -- what if the artifacts are designed to
steal Tiamat's power? Each artifact channels some of her power into a PC. As she diminishes, they grow into demigods.
White - Ice
Red - Fire
Black - Acid
Green - Poison
Blue - Lightning
Heart - Death
So.... Tiamat's actual body is a world. They have to embed the artifacts into her 5 heads and heart. When you embed the appropriate artifact, you steal part of her power. When all 6 are embedded, you become an ascended mortal (to demigod hood).
Aspect of Hate (White, Ice)
Aspect of Arrogance (Red, Fire)
Aspect of Destruction (Black, Acid)
Aspect of Betrayal (Green, Poison)
Aspect of Cruelty (Blue, Lightning)
Aspect of Death (Heart, Necrotic)
As a baseline (pre-ascension):
- You are immune to damage of that type
- Your weapon attacks deal an extra 1d12 damage of that type
- When you deal damage of that type, you gain that many temporary HP
- When you cast a spell that deals damage of that type, you deal an extra damage die and it is counted as a 1 level higher spell slot.
- You are immune to fear
In addition
Hate: If you someone hits you with a critical hit, you strike a critical hit, or you take damage that drops you below 1/2 max HP, you can go into a barbarian rage for 1 minute. While in this rage, you resist all damage (including the damage that triggered this effect, if any), you can still cast spells and concentrate, and you have advantage on all attack rolls of the creature who damaged you or you hit that triggered this effect. When you leave this rage, you suffer a level of exhaustion. You also regain a level of exhaustion on a short rest. You can enter this rage again while in it; this extends the duration, possibly adds another creature you gain advantage on, but also increases the number of levels of exhaustion when you leave it by 1.
(This is an improved costly version of barbarian rage. Note that I'm stealing from barbarian a lot, because you gots none.)
Arrogance: You have advantage on all saving throws. When you attack a creature or force a creature to make a saving throw on your turn, you can claim advantage on the attack or disadvantage on the save. If you do so, until the start of your next turn you become more vulnerable: all attacks on you have advantage.
(Note: this is a much improved version of reckless attack.)
Destruction: All damage you do is siege damage; objects and constructs take double damage, and you ignore damage thresholds. Whenever you score a critical hit, double the damage again. If you cast a spell that deals damage, on your next turn you can cause the spell to repeat as a bonus action at the same location; if it targets a creature (instead of an area) that is no longer at that location, there is a 50% chance you can redirect it to follow the creature, and 50% chance it instead targets the spot where they left. If you miss with an attack, you can move up to 15' and repeat the attack on a different target within 20' of the original target.
(Note: ouch. I hope whomever gets this smashes stuff good.)
Betrayal: You are destined to betray every creature you meet. Until you attack or force a creature to take a save, they are charmed by you and you have resistance to all damage they inflict and you automatically pass every save they force you to make. The first time you attack a creature or force it to take a save, you have advantage on the roll or they have disadvantage on the save.
(Note: This is worded carefully; creatures immune to charm are still impacted by sentence 2 and 3. Charm means you have advantage on charisma checks and they cannot attack you; it does not prevent spellcasting etc).
Cruelty: You cannot fail constitution saving throws, and concentration saves you force are at disadvantage. The first time on your turn you deal damage to another creature, it must make a concentration saving throw against the damage; if it fails the concantration save, it becomes paralyzed until the end of your turn, and you gain temporary HP equal to the damage you dealt.
(Note: Paralyzed is only on your turn, and end of your turn. Followup attacks by you, or readied attacks by allies, get a bonus. And flying stuff that can't hover plummets. Paralyzed is a decent "intense pain" effect, and also fits with lighting theme of blue dragons.)
Death: If you kill a creature and your HP is less than 1/2 of their max HP, you heal to that amount. You cannot die so long as another aspect is alive; effects that would kill you or destroy your body are ignored (power word kill, disintegrate, etc) even if any damage they deal is not. Whenever you take damage that would reduce you to 0 HP or lower, you are instead reduced to 1 HP and another aspect takes the remaining damage (their immunity or resistance can reduce the transferred damage). If another aspect is dead or dies, they are instantly brought back as an undead incarnation of themselves; their HP is restored to full, they are undead, and gain your cannot die and damage transfer feature (but only to still living incarnations), and cannot be restored to life even by a Diety. If all incarnations are rendered undead... you die as well, and your souls meld into an incarnation of Tiamat.
(Note: Death is a bit of a trap compared to the rest; the Heart of Tiamat is not something you can steal safely. I wouldn't tell your PCs about every clause here. The PCs that come back as undead; say you have the same memories, are loyal to Death, but you feel the aspect emotions ... a bit stronger every day.)
That looks like fun.
Only then, then you can fight Tiamat. In this reduced incarnation, she's merely the size of a small city, and each of you have to fight all of her heads at once; only once you defeat your head (which matches your aspect) can you help the others (Heart gets to gang up on one head).
---
For tactical complexity...
At the start of combat and at the end of each round, randomly 2 of her heads are Empowered until the end of her next turn. An Empowered head deals ~2x damage (does a breath weapon and a set of attacks?) and takes 10x less damage.
This should be obvious to the PCs and told about in advance.
Those not fighting empowered heads can thus drop them faster; as the non-empowered heads drop, there are fewer heads (so more likely to be empowered) and more PCs per head. So that sort of structurally guarantees a 6 on one or two teamup at the end.
In their ascended form, the PCs can have crazy stuff like a 60' fly speed, and where they fight Tiamat can have terrain to hide behind (on the empowered rounds). Her long necks make each head independently mobile.
