However, in the thread at the Ronin Army forums where I post links to my Freeport 5E articles, I was recently asked for help in converting Death in Freeport to 5E. That is definitely the adventure that I would choose to convert first--it's the one that started the whole setting and product line, and it's for 1st level and fairly short, so should be the easiest to adapt to 5E. My reply to that query started off with the caveat above, but then I tried to make a first stab at the problem. This column will reiterate and expand upon that initial answer.
First of all, Third Edition and 5E are different enough rules sets that I would never attempt to do an exact conversion. For some encounters (such as orcs and skeletons), it's easy enough to just use the 5E Monster Manual stat block. Human foes can be represented by the MM's low-level NPC stat blocks (acolyte, bandit, commoner, cultist, guard), possibly with some small tweaks to weapons and spells to better match the original.
The serpent people are the one real challenge in converting this module. For the degenerates, I compared reptilian humanoid races in the MM, and found that the troglodyte's ability scores and hit points were a surprisingly good match to the original 3E degenerate stat block. Keep multiattack, but replace their claw attacks with a single weapon attack and a shield (+2 AC). The troglodyte's chameleon hide is a decent fit for the 3E serpent people's racial bonus to Hide, so keep it, but delete stench and sunlight sensitivity. Finally, replace the Troglodyte language with Valossan.
For Milos, the sole civilized serpent person in this first adventure, the obvious quick-and-dirty option is to use the cult fanatic stat block (which already has the correct 2nd-level spells!), and add darkvision, Valossan, and the doppelganger's shapechanger ability.
Converting serpent people from later adventures follows a similar process. First, find an existing NPC stat block that approximates the serpent person's class and level. (Volo's Guide to Monsters greatly expands the choices here, including many new arcane specialties for spellcasters.) For civilized serpent people, simply add darkvision, Valossan, and shapechanger, as with Milos. For more powerful degenerate serpent people, more changes will be needed due to their additional racial traits and their low Intelligence and Charisma scores. See Chapter 9 of the Dungeon Master's Guide for more guidance about adding racial traits to NPCs. Use the Troglodyte entry of the NPC Features table--with the changes given above--to modify an existing NPC stat block.
Two other concerns that I did not address in that quick reply include skill and save difficulties, and treasure.
5E was designed with bounded accuracy, meaning that modifiers to attacks, saves, and skills do not increase as rapidly, or go as high, as in 3E's open-ended structure. The DC for a skill check or saving throw will tend to be a little lower in 5E, though at 1st level, that difference is minimal (a point or two in most cases). For DCs that aren't set by a creature or PC's stat block, look for examples in the rules for appropriate DCs for the desired difficulty of the task. In 3E, DCs started to get challenging at 15; in 5E, that's probably more like a 12. A DC 20 task in 3E probably should only be a 15 in 5E, and so on. Also note that many skills have changed name between editions, and a few were dropped entirely; for example, a Gather Information check should now be a Charisma, or perhaps Charisma (Persuasion), check.
Purely monetary treasure seems to be roughly equivalent between the two editions, but 5E is designed to make magic items far rarer. Few if any assumptions are made about what gear a character will have at a given level, unlike 3E's finely-tuned "wealth by level" track. And because magic items are harder to acquire in 5E, it's also harder to simply buy them--which means that character have significantly fewer ways to spend their acquired wealth than in 3E. But keep in mind that because the Freeport setting was born during the 3E era, most past titles assume that magic is fairly common and that low-powered magic items are easy to acquire. Because of this, DMs of Freeport campaigns may want to allow the DMG's optional downtime activities related to crafting and selling magic items. Uncommon and rare items should remain scarce--and in a hive of scum and villainy like Freeport, they may quickly attract unwelcome attention from others who covet them. Death in Freeport has a number of low-level magic items in it, but in my opinion, it's not an excessive amount for a 1st-level 5E adventure. Later adventures in the setting will need a more careful vetting of magic items, as 3E's assumptions about wealth become more pronounced at higher levels.
Finally, here's a summary of my quick conversions for characters and creatures encountered in Death in Freeport:
- Sailors: Pirates count as Bandits (MM); change weapons to clubs, and add Intimidate +2. (Note that this press-gang initially attacks to knock out characters at 0 hit points, not kill them.)
- Egil: Acolyte (MM).
- Orc Pirates: Orc (MM).
- Aggro: Orc (MM) with extra hit points, or possibly an Orog (MM).
- Captain Scarbelly: Orog (MM) with a +1 battleaxe. (For the staff of defense in his ship's hold, see New Magic Items in The Lost Mine of Phandelver, in the D&D Starter Set.)
- Yellow Shields: Cultist (MM); add shield (+2 AC).
- Belko: Apprentice Wizard (Volo's).
- Rittoro: Guard (MM).
- Enzo: Enzo is not given stats in the adventure. If needed, he is a commoner (MM).
- Camouflage Pit Trap: Hidden Pit (setback, 1d10) (DMG).
- Degenerate serpent people: Troglodyte (MM); remove claws, stench, and sunlight sensitivity; add a spear attack and a shield (+2 AC); change language to Valossan.
- Skeletons: Skeleton (MM).
- Temple Attendant: Acolyte (MM) with some spells changed.
- Milos: Cult fanatic (MM); add darkvision, Valossan, and shapechanger (as doppelganger, MM).
- Lucius: Apprentice Wizard (Volo's); remove spellcasting; add proficiency with Religion, calligrapher's tools, and several languages.