Rules/Restrictions:
Any race published is allowed. You may use the Customise Your Origin option from Tasha's Couldron of Everything. Any officially published Class by WotC is allowed as well as Optional Class Features. No Unearthed Arcana.
If there is a circumstantial bonus, such as Assassinate or Haste, please show each step in the damage calculation.
No magic items, charms, blessings, or epic boons.
Exception regular: +1, +2, +3 weapons
No, the "damage done" can't be to yourself.
You are on your own: there is no other PC nor an NPC.
Take into account that you might not surprise the Tarrasque.
You do not have advantage by default. If you can generate some advantage for yourself, then you may do so. Explain how you do it.
You are up against an Tarrasque. It will make use of its abilities including Legendary Resistance and Legendary Actions, if it has the chance to do so. All of its charges are full. Magic Resistance and Reflective Carapace work.
Link to the Monster Stats Block
Clarification: You can use any Race, Class, Spell or ability for the official published WotC books. Anything not listed in those book is not allowed. These are:
- Basic Rules (BR)
- System Reference Document (SRD)
- Player's Handbook (PHB)
- Sword Coast Adventurer's Guide (SCAG)
- Xanathar's Guide to Everything (XGtE)
- Guildmasters' Guide to Ravnica (GGtR)
- Mythic Odysseys of Theros (MOoT)
- Wayfinder's Guide to Eberron (WGtE)
- Eberron: Rising from the Last War (E:RftLW)
- Explorer's Guide to Wildemount (EGtW)
- Dungeon Master's Guide (DMG)
- Tasha's Couldron of Everything (TCoE)
If you have to roll a die for anything the outcome is random so statistic rules.
If you have the Longstrider Spell you may cast it in advance of the set-up round.
You start with one non-combat round for which you are more than 120 feet away from the Tarrasque. In this round you may, for example, buff yourself or summon other creatures with spells. But you can't enter the 120 feet radius around the Tarrasque. Initiative is rolled at the beginning of the combat round after yourthe non-combat round. One full combat round occurs, so you can consider the Tarrasque attacking back if it can do so. It will use its Chomp with Swallow. For the Multiattack it will also use its Swallow action.