Most commonly Dungeons & Dragons tends to be sequences of role-play, exploration, broken up by combat situations and the occasional shopping trip. There’s variety in the proportions of course, but there are some very rare opportunities for the DM to call for initiative where an actual fight is unlikely because there are more important things to worry about.
I’ll be using rules and skills for D&D 5th edition (5th edition is best edition, all hail 5th edition) but will be left loose enough for you to modify the rules for different systems and editions to suit your campaign.
It came from beneath and took a bite out of the hull! Apparently a mouth full of gunpowder was enough to give it pause for thought, but there’s no staying on board now. Lifeboats are finite, prisoners have escaped, and there’s a treasure worth dying for in the captain’s cabin. A lot of things are about to happen very fast, roll initiative! For a medium sized ship of 80 ft. in length give the players approximately 10 rounds (about a minute in real time) before they are pitched into the sea, or trapped below decks to drown.
Movement: Athletics and acrobatics checks are going to be an essential for navigating the wildly bucking boat as water floods the lower levels, barrels of oil, coiling ropes, and madly dashing crewmen are going to make life harder and harder. After round 5 if players are still on board, they should be making strength and dexterity saves every time they start their turn outside as they avoid falling and rolling debris, or take 1d8 damage and be knocked prone as they cross.
Evacuation: Let’s say there are half as many lifeboats as there are crewmen and passengers. Persuasion and deception checks made to convince crew that they should go down with the ship will be near-impossible, say DC 21 may result in a begrudging act of self-sacrifice. Intimidation will be more successful – DC around 16 as their life is on the line one way or the other – but the simplest way to oust someone from a lifeboat will be a competing strength check to throw them overboard.
Those failing to secure a space on board will have to resort to alternative methods of survival. Perception DC 12 to find an adequately sized piece of flotsam, empty barrel, or something else to cling to. Alternatively a survival DC 13 check will help a creature fashion an emergency raft or floating device hastily out of nearby materials like rope, crates, or other paraphernalia.
Save the MacGuffin: The captain has been protecting the map/magic compass/plot device that will prove essential to furthering the plot. Alternatively there was a safe in her chamber and you entrusted her with your valuables. It will require a strength check to bust down the door, around DC 13, or a DC 12 check with a set of thieves tools to open the lock. Investigation checks will be difficult, around DC 14 (with each attempt taking a full round) to find where the MacGuffin is hidden. It may require another check with thieves tools to open the chest/safe/lockbox, DC 15 (one full round per attempt).
Revenge of The Imprisoned: Your captives are set loose by the attacking monster, and seek revenge. If anyone was guarding the prisoners they will be the first target, otherwise they’ll find whoever they can as they make their escape. With snatched up rope, manacles, and whatever weapons they can steal along the way, they will attempt to tie the heroes or crew to the boat somewhere below decks so that they drown. If your prisoners do not already have stats then utilise a level-appropriate creature from the Monster Manual, like a thug, bandit captain or a gladiator but removing the weapons. They will attempt to make competing strength checks to capture player characters and crew and leave them to die while they make their way to a life boat. Ropes will require a DC 15 strength check to burst or a DC 13 dexterity check to wriggle free.
Trapped Below: From round 5 onwards the decks below have begun to flood. Athletics checks made to swim through the flooded decks have disadvantage for the flotsam and detritus in the way and will be DC 12, and in the panic of the situation a creature will have to make a constitution save DC 12 after a single round under water or suffer 1 level of exhaustion. Success on any save will mean the creature has kept their composure and their breath for at least 1 minute of holding their breath, but each failure will result in another level of exhaustion and having to repeat the save next round.
This is no mortal foe, there is no blood that can be shed and no flesh to assail, but it makes the beast no less powerful and capable of reaching into our world to strike us down. Across the many planes of existence there are plenty who covet that river of souls, the font of life, and hub of all the swirling powers and energies, the material plane, and many are too unreal to wholly exist within its confines. The great Fiend that is trying to crawl its way into the world can be stopped and its influence ended, only by intrepid souls willing to brave its wrath. The space of the ritual may be a large compound like a cathedral, town, or cavern network, and players must negotiate it while the Fiend assault them from beyond.
Totems: Around the ritual site cultists have fashioned totems of power designed to anchor the Fiend’s power, to create a stable place to anchor it into the world, columns of stone carved with arcane inscriptions. An arcana check of DC 14 will give players a complete idea of what countermeasures can be taken against it. Proficiency with mason’s tools will make the totems easy to destroy, a DC 12 check to deface the carvings enough to make the stone mundane. Attacking the totem will be harder, it has an AC of 12 and a total of 30 damage will need to be dealt to it, it is immune to poison and psychic damage, and resists all other damage. Spells such as dispell magic and antimagic field will suppress the totem for the duration.
The Font: Finding the centre of the Fiend’s power is not easy, detect magic will give advantage on investigation and perception checks to follow the flowing energy from the Totems. Finding it without magical aid will require a DC 15 investigation check to follow runes, the footprints of cultists, or insight to observe the growing intensity of the Fiend’s countermeasures. Three successful checks will be required to find the font. The font may be a dark tome of lore containing the true name of the fiend, or a mortal sacrifice whose open chest cavity reveals a burning heart, and any single effort made to destroy it will complete the ritual, but not until at least three Totems are destroyed.
The Fiend Fights Back: At initiative count 20 and 10 the Fiend will make counter attacks, one against any single creature within 10 ft. of a totem, another against all creatures within 60 ft. of its Font, and one against a random other creature. Creatures within 30 ft. of the Font have disadvantage on saving throws against the counter attacks. Roll 1d4, the same attack targets all creatures:
1- Fire and Fury: The creature is assaulted by a vision of flames, and must succeed on a DC 13 Charisma save or it takes 1d8 fire damage and makes an attack against a creature within 5 ft. of itself, or half damage on a success and it makes no attack.
2- Teeth of the Wind: A swirling vortex of shards engulf the creature, it must succeed on a DC 13 Strength save or take 1d6 slashing damage and be knocked prone, or half damage on a success and it remains standing.
3- Swarming Nightmares: Insects by the thousand rise to assault the creature where it stands, it must succeed on a DC 13 Dexterity save or take 1d8 piercing damage and drop whatever it’s holding, or take half damage and not drop anything on a success.
4- Tempest: Arcs of power race towards anything that lives. The creature must succeed on a DC 13 Dexterity save or take 1d8 fire damage and 1d8 lightning damage, or half damage on a success.
Each of those proved longer than I expected, I will cut this short with the option to revisit it again one day, I have others. Next week, perhaps something shorter, but you get to pick which: