Death's Door

In the Stranded Sailor, enter Jefferson the squid's boat and choose "Yes" when he offers you soup.

With the Bomb, blow up Grunk's wall in the Mushroom Dungeon and talk to him, then speak to him again at the Stranded Sailor where he becomes your devoted fan.

Buy 12 total stat upgrades from the big crow vendor in the Hall of Doors with souls. Farm souls if you're short.

Beat the game - defeat the three giant-soul bosses (the Urn Witch, then the Frog King / Lord of Doors chain) to reach the standard ending. Unavoidable if you finish.

Defeat the optional mini-boss inside the Urn Witch's mansion (right arch, bird-cage elevator, cobweb door) to upgrade the Flame spell.

Buy all 5 levels of any one stat (Strength, Dexterity, Haste or Magic). Strength is usually cheapest to max first.

In the Lost Cemetery, bomb a destructible doorway and beat the mini-boss inside to upgrade the Bomb.

In Stranded Sailor, hookshot to an island with a lever, pull it to open a gate, and beat the mini-boss to upgrade the Hookshot.

Collect all 5 melee weapons: the starting Reaper's Sword, the Discarded Umbrella (Hall of Doors staircase), Rogue Daggers (Urn Witch estate mini-boss), Thunder Hammer (Mushroom Dungeon), and Reaper's Great Sword (Stranded Sailor, after hitting 4 switches).

In the Lost Cemetery, complete the fire / brazier puzzle rooms (needs all three other abilities) and beat the mini-boss to upgrade the Arrow.

In the Urn Witch estate, solve Pothead's pot-breaking puzzle - break the pots in the right order (the arrows help) and follow the statue path to the hidden garden.

After defeating Betty, go to the Stranded Sailor, talk to Barb, and choose "Yes" to hear her magnum opus.

Post-game. Ring the bell in the Lost Cemetery tower to turn it to night, collect the teddy bear from the docks, and bring it to Jefferson - he then rides on your back and directs you to a hidden tablet.

During the Grandma boss fight, wait for her to tip over and spin, then fire your Flame spell at the exposed pot lid. A one-shot during that fight only - if you miss it, you'll have to replay.

Find and pray at all 16 shrines (8 Vitality + 8 Magic) across the world - each permanently boosts HP or magic. Use a map for the well-hidden ones.

Find and talk to all three owls - in the Urn Witch estate, the Overgrown Ruins, and the Old Watchtowers. Each gives a tablet piece toward the true ending.

In the Grove of Spirits, the little green forest spirits trail you as you pass them - gather 10 or more at once (combine the groups before and after the boss).

Find all 50 Life Seeds throughout the levels and plant them in the empty pots dotted around the world. The Rusty Garden Trowel helps you spot pots.

The secret true ending. After 100%ing the game, collect all 7 Ancient Tablets of Knowledge (via the owls, spirit puzzles, braziers, planting every seed and more), defeat the Grave Digger, then enter the final door at the Camp of Free Crows.

Collect all 24 Shiny Things hidden across the world - several need all four magic abilities (so post-game access). A location guide is strongly advised.

Reach 100% completion: all 50 seeds planted, all 16 shrines, all 24 shinies, all 5 weapons, all four ability upgrades, the Grave Digger beaten, and the owls' tablet. The game tracks the percentage; hit 100% and it pops.

During the Frog King's second phase, when he opens his mouth from the arena wall, throw a Bomb straight into it. A one-shot during that fight only - replay if missed.

Get a forest spirit to follow you in the Hall of Doors (one's on a bench near the Urn Witch return door), then lead it through the metal-detector scanner at the hub entrance so it trips the alarm. Best done before the final boss - the NPCs relocate afterwards.

Grab the Discarded Umbrella early (Hall of Doors staircase), equip it, and never use any other melee weapon for the whole playthrough - magic and ranged attacks are fine. Swap melee even once and the run is void for this, so plan it from the start.