There are eight nine total endings possible in Strange Antiquities. Some of these you can get by reloading your save on Day 18, while some you can only get based on certain choices you made early on. For ease of reference, I've left the days of the important decisions unspoiled in this guide. If you're trying to achieve all endings, don't forget that the game autosaves each day, so you can go back and reload your file on the day of an important decision.

I haven't given the answers regarding the items to be used within the endings themselves, nor regarding the plinths in the shop. Use the in-game hint system if you get stuck, or else comment below and I'd be happy to help!

This guide is all my own work. Please credit me or link back here if reposting elsewhere. Thank you!

Day 15: Give Verona Green the Zephyrim when she visits the shop.
Day 18: Eli will come to the shop at the end of the day and tell you where to meet him. Go there and assemble the Pillar of Calaris using the note Eli had given you.
When you have the choice between the Wyxic Cord and the Casket, choose the Casket.

Day 5: Give Rebecca Rose the Aries Remnant.
Day 13: Give Barton Potter the Babellia.
Day 18: Go to the location Barton tells you to and play through the scene. (If you missed what he said, you can see dialogue history in the character profiles)

Day 12: Give Audrey Bell the Fortinance when she visits the shop. She will return a couple days later and give you a note about the Sacrament
Day 18: Go the location referenced on the Sacrament note. Assemble the correct items using the same note. When you have the choice between St. Cedric's Thunder and Corvan's Gift, use St. Cedric's Thunder.

Day 12: Give Audrey Bell the Fortinance when she visits the shop. She will return a couple days later and give you a note about the Sacrament
Day 18: Go the location referenced on the Sacrament note. Assemble the correct items using the same note. When you have the choice between St. Cedric's Thunder and Corvan's Gift, use Corvan's Gift.

Day 5: Give Rebecca Rose the Spirit Eye.
Day 13: Give Barton Potter the Babellia.
Day 18: Go to the location Barton tells you to and play through the scene. (If you missed what he said, you can see dialogue history in the character profiles)

Day 15: Give Verona Green the Zephyrim when she visits the shop.
Day 18: Eli will come to the shop at the end of the day and tell you where to meet him. Go there and assemble the Pillar of Calaris using the note Eli had given you.
When you have the choice between the Wyxic Cord and the Casket, choose the Wyxic Cord.

Day 15: Give Verona Green the Abrasive Mether when she visits the shop.
Day 18: Eli will come to the shop at the end of the day and tell you where to meet him. Go there and assemble the Pillar of Calaris using the note Eli had given you.
When you have the choice between the Wyxic Cord and the Casket, choose the Casket.

Day 15: Give Verona Green the Abrasive Mether when she visits the shop.
Day 18: Eli will come to the shop at the end of the day and tell you where to meet him. Go there and assemble the Pillar of Calaris using the note Eli had given you.
When you have the choice between the Wyxic Cord and the Casket, choose the Wyxic Cord.

Yes, there is a secret 9th Ending! Note that you can technically get this ending at any point in the game provided you have the right items. It's probably easiest to reload from a Day 19 save. lovesthesoundof posted a helpful step-by-step hint guide in the comments here, which I've replicated here for ease of access:

Originally posted by lovesthesoundof:

1: Decipher the secret of the artefact guide book. Hint: Someone has written on some of the pages... What could it mean? Answer: Look for the three Xs written on page corners. The three items you need are a Babellia, an Orford Spire and a Thaumic Needle. You know what to do with sets of three items by now, don't you? ;D

2: Decipher - TRULY decipher - the secret of the clock. Hint: It seems there's a pattern to the way the hands point... Could that help you bypass the broken setting and find the vault code? Answer: First a trio of neighbours, then a T shape, then a Y shape. The broken setting is a Y shape. The two stationary hands have a space between them; the spinning hand should be pointing opposite that space. That's the code you need for the vault.

3: Decipher the secret of the symbol. Hint: There's a unique symbol on the lock of the final vault. It doesn't seem to be in the Symbology book... Have you seen it anywhere else? Answer: It's in the catacombs. The tomb in question is in the upper left of the map and is safe to click.

4: Decipher the secret of the past. Hint: Do you have something that helps you read what was once there? Answer: Use a Carrick's Candle at the shopkeeper's tomb.

5: Decipher the secret of the heart. Hint: The shopkeeper's riddle tells you how to unlock the final vault. You have at least two of the three tools you need...but what could Sweetbreath be? Players of the previous game might know where to look... Answer: A Sextrum will guard your mind, and a Sleep Vessel will keep you wakeful. Sweetbreath refers to the Palliance plant, and you can get one from a visit to Strange Horticulture - #2 on the Undermere map. All of these items can be in the shop as early as Day One. Place them in the locks on the final vault and enjoy the secret ending!

Hope this helps! :D

Other decisions you make throughout the game can affect individual characters even if it doesn't change the ending. While most characters' fates are tied to the ending you get, listed below are the characters whose fates can be affected individually. Spoiler text here will have story-based spoilers!!

Day 5: Aries Remnant. Assuming she survives, Rebecca is grateful to you. Locks you out of Ending V
Day 5: Spirit Eye. (**needs more testing**) Assuming she survives, Rebecca is ready for another adventure. Locks you out of Ending II

Day 7: Witch's Finger. Mr. Hungerford is on the road to recovery (Whether he actually lives depends on the ending you achieve).
Day 7: Restful Mether. Mr. Hungerford dies.

Day 8: Grey's Grace. Leads to another decision on Day 9:

• Diverging Hook: You can recover what Bonnie took from you. Bonnie ends up a shell of her former self.
  
• Corvan's Might: Untested, but I assume this leads to the same result as if you had given it to her Day 8.
  
• It occurred to me that you can presumably just ignore Bonnie, never meet her in the woods nor give her either item. However, I assume this means Jupiter remains missing the rest of the game. If anyone that's less of a baby about fictional pets than me is willing to test this please lmk @Shirato checked and you can't end Day 9 without doing one of the two above options!

Day 8: Corvan's Might. Barton's appearance changes and he has an extra interaction with you, but his story remains unchanged otherwise. This may change Barton's fate--see his section. Bonnie goes missing.

Day 12: Morgan's Folly. Audrey dies. Locks you out of Endings III and IV.
Day 12: Fortinance. As far as I can tell, Audrey will die in every ending except Ending III.

Day 13: Pitt. Barton dies UNLESS you gave Bonnie Corvan's Might (thanks, lhb!). Locks you out of Endings II and V
Day 13: Babellia Barton's ultimate fate depends on which ending you get

Day 17: Dread Vivant Thaddeus survives the night; his ultimate fate depends on your ending
Day 17: Lunima Thaddeus dies.

Day 8: Giving Bonnie Corvan's Might leaves Jupiter unaffected. Giving her Grey's Grace will lead to Jupiter not being present in Day 9 and the following happens:

• Day 9: Either way you deal with Bonnie, you will find out where Jupiter is and can go rescue him. I think you can also choose NOT to rescue him. But like... why would you...

I will say: although the text differs depending on your ending, Jupiter should be okay no matter what choices you make!