I was pondering the logistics of fighting properly huge creatures and I wondered whether there was anyhting people have used to make it more than trying to make atatcks whilst avoiding being hit!

I am wondering if a Shadow of the Collossus style system for immense bosses would be a good thing to work with - the boss becoming a moving battlemap with ability scores needed to hold on, and vulnerable parts which need to be identified and exploited to have any hope of bringing the beast down.

Has anyone ever run anything like this? Whilst I love the imagery of a tarrasque, I sometimes feel like it would be less than epic to fight one - and I know thay're supposed to be more of a plot device (IE, "Stop the tarrasque" rather than "Kill the tarrasque", where you can lure it away or put it to sleep or whatever you need to do), but if I were to fight one, I'd be wanting to make it a really epic fight!

I may start pushing some rules together for this very thing - Anyone got any ideas or ways thay've run such things?

one way would be to give each individual limb (arm, leg, tail, head) its own AC and hp stats, if the pc's focus their attacks on one part of the creature, they could (in theory) disable that appendage. If a titan or other bipedal creature, then the loss of a leg could cause it to fall prone. The hp of the creature as a whole would be lowered as a result, either by the damage done to the limb or a percentage of that damage. Example if the left leg hass 100 hp and the party manages to take that to 0, either subtract 100 hp from the creatures total or just 25% of the legs hp from the total(25hp). 25% is just an example # you could do more or less. I have done this in the past and it has worked for me. Also with creatures that regen hp, not all of the regen hp goes to the total, some goes to damaged appendages. Hope that helps.

If these things are going to be truly titanic - e.g. a mile tall, then the PCs will need to employ unorthodox strategies and defeating it should be the end of an epic quest chain rather than a one-off battle. Little less than a Meteor Swarm is even going to dent it... and even then those meteors are going to be mostly bouncing off it. You'd need a whole army of casters hurling 9th level spells to even slow it down. Physical attacks? Total immunity, the Titan's armour is thicker than the walls of most houses, and harder. This behemoth cannot be stopped by ordinary means. It cannot be fought. It annihilates all in its path. Imagine those scenes in Neon Genesis Evangelion where the army throws everything it has at the Angles and they just shrug it off. 

• Let the heroes see the sheer futility of trying to stop it crushing a city. Let them throw everything they have. They are less effective than ants.

But wait!

• Our heroes fight their way through Hephaestus' Forge, and uncovered the titan's schmatics.
• They venture to Mount Doom and gathered enough adamantine to cast the master drill
• They convince the Red Dragon of the Volcanic Realm that his lair would be under threat, the dragon agrees to carry them into battle
• The PCs land on the titan's head behind its neck and deploy the drill. Hold off its defensive Anti-Body Constructs.
• Drilling in, the PCs effectively enter a dungeon within the titan and destroy it from the inside

I was considering converting Allabar, the Opener of the Way into 5e (it's a 4e monster that is... a aberrant living planet, or at least a planetoid), and my thought there was to treat the body as a lair/domain which you have to make your way through (dealing with 'antibodies' and the like) before reaching the core, which is actually something on a scale that PCs might consider damaging. You can do similar things for things like a Warforged Colossus.

Quote from Sanvael >

• Our heroes fight their way through Hephaestus' Forge, and uncovered the titan's schmatics.
• They venture to Mount Doom and gathered enough adamantine to cast the master drill
• They convince the Red Dragon of the Volcanic Realm that his lair would be under threat, the dragon agrees to carry them into battle
• The PCs land on the titan's head behind its neck and deploy the drill. Hold off its defensive Anti-Body Constructs.
• Drilling in, the PCs effectively enter a dungeon within the titan and destroy it from the inside

 That is an awesome idea! 

There is one horrifying story I read where in the center of hell there is an ancient God/Titan that has been hollowed out while still ALIVE and the demons live inside him, using his body as fortified castle.

Absolutely the best/worst description of Hell I have ever read.

I also like the idea of treating it more as a Lair than as a traditional creature. Then the challenge is more about surviving and reaching weak points, and reacting to Lair Actions than fighting a creature that can be easily targeted with attacks each round. Then it could have lair actions like trying to shake them off, or swiping at characters with a limb, or just shifting position and changing the angle that players are moving.

The hard part is making sure that the players can't just, y'know... fly directly to the weak points and skip this whole challenge

Quote from TransmorpherDDS >>

The hard part is making sure that the players can't just, y'know... fly directly to the weak points and skip this whole challenge

 That's why you don't put the weak points on the surface. It's a problem for intermediate scale creatures, though; going inside the monster the size of a mountain is easy to figure out, going into the monster the size of a castle isn't that unreasonable, but it doesn't work for creatures on the puny scale of things like the Tarrasque.

Matt Mercer had the party face a titan in the first campaign of CR, they didn't actually fight the Titan, instead they got into it and fought through it getting to the BBEG who was riding it like a giant mech. But this is something you could borrow from, maybe to kill the titan they need to fight their way into it's body and deal with the monsters that reside in it, or it's natural defences. Maybe they need to find and destroy the heart, or just do enough internal damage it slows down and stops going back into a long slumber. 

Quote from BigLizard >>

I like the concept that was introduced in Forbidden Lands where when you fight a big boss-like monster, they act on multiple initiatives.  So a really big beast might have three separate initiatives where they act.  I tried it in my 1e game and it worked really well, I'm fairly certain it would work just as well in 5e.

Legendary actions do this to some degree (what degree depends on what LAs the creature actually has).

If you think about Shadow of the Colossus, the fun comes from figuring out the puzzle of each colossus. The actual stabby part is totally incidental. Figuring out your approach, figuring out the weak spots, figuring out how to stay under its radar or avoid its retaliation. In short, making a plan and then executing it. The "making attacks to reduce it to 0 hp" part should not really be the focus IMO.

Quote from scatterbraind >>

If you think about Shadow of the Colossus, the fun comes from figuring out the puzzle of each colossus. The actual stabby part is totally incidental. Figuring out your approach, figuring out the weak spots, figuring out how to stay under its radar or avoid its retaliation. In short, making a plan and then executing it. The "making attacks to reduce it to 0 hp" part should not really be the focus IMO.

This is a good point. I feel like, after completing an epic plan to get to the heart of the monster, it would be pretty rubbish to have the party then have to deal X damage to the heart - just stabbing it would probably suffice!

Though having epic battles occur when they reach the key place - such as enemies guarding it - would also be an important part. But I agree that the challenge of ye big beasty should be in making their way to the weak parts, working out any rhythms of the game (I intend to use dynamic battlefields, like when the colossus' shake in shadow of the colossus,  and with body parts moving past one another. It's going to be hell to work out, but I will do it!).

The inside of the Titan shouldn't be the only thing that's heavily guarded. Maybe it has pores that flying minions crawl out of. Perhaps it's got cannons or acid-spewing mouths on its hide. Whatever you decide it to be, there should be some threat to going near it other than the risk of being stomped. This makes the battle to get inside the Titan really tense and could make for a few encounters while the adventurers scour its body for a potential weak point.