Titanic Encounters System

==Titanic Encounter Basics==
Any creature determined by the DM as Titanic (usually Gargantuan-Sized and above) prompts a special combat mechanic named Titanic Encounter. These rules are specifically designed to regulate what these creatures can do because of their sheer size and to give precise rules on how climbing them works. 
In order to engage a Titanic Creature, you enter its space while moving or as a Reaction to being the target of a Melee Attack from the creature. This usually requires some form of check (e.g., a Strength (Athletics) to jump onto the creature from above or a Dexterity (Acrobatics) to grab on as it flies past). This check is contested by the Titanic Creature’s Dexterity (Acrobatics) check to avoid the little pests.
Being engaged with a Titanic Creature usually means that you are physically on the creature in some way. Being engaged is an exception to the normal movement rules preventing you from willingly ending your move in another creature’s space. As long as you are engaged with the creature, you will be carried with it when it moves.
Characters who are not engaged with the Titanic Creature suffer Disadvantage on their Melee Attacks targeting it. Characters engaged with a Titanic Creature gain Advantage on their Melee Attacks targeting it.
==Titanic Creature Actions==
Shake Loose - As a special Melee Attack, a Titanic Creature can attempt to shake off anyone currently engaged with it. When they do so, all engaged characters must succeed on a Strength (Athletics) or Dexterity (Acrobatics) Skill Check contested by the creature’s Strength (Athletics) Skill Check. On a failure, the engaged character is shaken loose into an adjacent space of their choice and is no longer engaged with the Titanic Creature. They must also succeed on a DC 15 Dexterity Saving Throw or suffer 2d6 Falling damage (or more if the creature was flying at a height greater than 20ft).
Siege Monster - All titanic creatures are given the Siege Monster ability. They deal double damage to objects and structures.
Titanic Destruction - All Titanic Creatures are given the Titanic Destruction Ability. They can deal damage to structures within reach as a Bonus Action.
==Titanic Creature Rules==
Flying Characters - Flying characters and characters on flying mounts do not suffer Disadvantage when making Melee Attacks against a Titanic Creature. However, if a Titanic Creature dashes by them, they must succeed on DC 15 Acrobatics Skill Check or begin falling down as the winds created by it's sprint swats them away.
Engage with Vantage - If circumstances allow a character to attack a Titanic Creature’s vulnerable areas, then, at the DM’s discretion, it will not be necessary to engage with the creature in order to avoid suffering Disadvantage on Melee Attacks against it. For example, a character might be located on a wall or in a tower that the Titanic Creature is passing by, or they might be riding another Titanic Creature. Such vantage points do not grant Advantage on attacking the Titanic Creature, but may grant Advantage on Skill Checks to engage the Titanic Creature.
Hey! I Was Standing On That! - When a Titanic Creature destroys a structure that a PC is standing on, they can attempt a DC 15 Dexterity Check as a Reaction to land on the creature and immediately engage it.
Helping Others to Engage - This can be done with a standard Help Action, granting Advantage to the aided character’s Skill Check to engage the Titanic Creature.
Knocked Off - If a character is knocked off a Titanic Creature, the DM may allow them a DC 12 Dexterity Saving Throw to grab hold of a lower point on the creature and remain engaged. In addition to deliberate enemy action , the DM might also call for such Saving Throws (possibly at a higher DC) in response to environmental conditions — when the Titanic Creature smashes through a wall or flies through a waterfall, for example.