In my previous posts about converting monsters from 2e and 3e to Fifth Edition, I mention that many monsters have been converted already. I mentioned that to dissuade you from feeling like there's a need to sit down and struggle through the math to convert your favourite monster, at least not before checking to see if it (or something like it) has already been converted for you. The problem with that, though, is that sometimes a monster changes significantly from edition to edition, or else one edition opts to provide a high-level version of the monster while the other provides a low-level version. For that reason, I find it exciting when I find a book with an edition for 3.5 and 5e concurrently. When such a thing happens, you can compare the exact same monster with itself, as it appears in a parallel edition universe.
I happen to have both the 3.5 and 5e versions of the Frog God Games book Tome of Horrors (acquired from two separate Humble Bundles, incidentally), so I in this post I am going to review Frog God's conversion of the Amphisbaena.
A quick note: I refer to 3e, Third Edition, 3.5, and Pathfinder interchangeably because in terms of lineage, spirit, and math, there's essentially no difference, and in real life, I use books from both 3.5 and Pathfinder.
The amphisbaena isn't as famous as the Beholder or the spectrum of chromatic and metallic dragons, but it's a monster created by Gary Gygax for the First Edition of D&D. I never had a Monster Manual for 1e, so I don't know its history first-hand, and it didn't make it into the official 5e Monster Manual. Frog God Games has permission to print the beast, though, and that's how I know of it. In its modern third-party form, it's a Large snake with a head at both ends of its body (so, its tail is another head). Its most unique trait is its ability to survive being split in two. If you cut an amphisbaena in half, both halves function as one-headed snakes, eventually growing the missing half of its body back. Besides that, its bite carries poison that requires a saving throw to resist.
Those are the defining traits of the monster, so those are likely the skills one expects to see in a conversion from a previous edition to the latest. In theory, everything else is "just" math.
The 3.5 amphisbaena has AC 17.
17 * 0.8 = 13.6 (round up to 14)
The 5e Tome of Horrors version of the beast ranks it at AC 15, so allowing for 1 point of artistic license or playtest feedback, the AC conversion is basically by the book. It would be interesting to know whether the extra 1 point was from playtest feedback, gut instinct, or artistic license, but only as a matter of curiosity.
There is no magical beast species in 5e, so the amphisbaena is classified as a monstrosity (that is, a monster). Its size remains Large and its alignment remains Neutral (actually, "unaligned) in 5e, indicating that the creature makes no ethical choice, and so cannot be said to be even neutral). No surprises here.
The 3.5 version has HP 42, from 5d10 hit die with +10 plus 5 points.
The 5e version has 60, from 8d10 plus 16.
The 3.5 version is based on monster statistic charts, the most updated of which appears on page 291 of the Pathfinder Bestiary. This table provides nearly all scores for a monster, including AC, HP, DC for primary and secondary skills, and so on, all based upon the target challenge rating (CR). The amphisbaena is CR 4 in 3.5, so that dictates, among other things, the HP.
Hit die are dictated by the Creature Hit Dice chart on the same page of the Bestiary. For a CR 4 magical beast, the table lists 5 hit die. The extra points presumably work to ensure an appropriately high HP, although I wasn't able to find documentation on where the 10 and 5 bonuses come from.
The same basic principle applies to the 5e version of the monster. Tables in the Dungeon Master's Guide dictate hit die and HP and CON bonuses. Interestingly, however, the 5e amphisbaena is only CR 3, one level below the 3.5 edition. I believe this reflects the bias toward lower levels that 5e imposes, probably in an effort to help the math scale up to level 20, but that's only a guess and I have no math or documentation to support that.
The ability scores, unsurprisingly, stay exactly the same between the two versions. I would have only expected ability scores to change in the event that the creature's actual abilities changed (for instance, if the new version of the amphisbaena was based no a rattlesnake and lost its Stealth ability, it wouldn't surprise me to see less DEX).
I'm not sure, but the Tome of Horrors version of the amphisbaena lists no skill bonuses. Entries in the Monster Manual do, so it's an interesting omission. My guess is that the designer figured that with a basic monster like the amphisbaena, it just made more sense to let the DM add bonuses to skills as required by the game session. After all, aside from having two heads, it's just a snake, and it's safe to assume that everyone's familiar enough with a snake to know what roll an amphisbaena would reasonably have bonuses for. Making safe, common sense assumptions like this seems very 5e in spirit, to me: give the DM more whitespace, less to read, and allow for improvisation instead. If you like the security of rules in your games (as I generally do), this may seem a little sloppy. After all, who's to say a particularly poor DM won't assume snakes can slither up walls, or swing from one ledge to the other using its suddenly prehensile mouths, or other silly feats? I prefer for my emulated world to be well-defined, but 5e places a lot of trust in its players, and ultimately my "concerns" are theoretical. I've never had a poor experience playing D&D 5e, so I don t blame the designers for keeping the system light and relatively easy.
Other details are dropped. For example, there are no Fortitude or Reflex or Will saves in 5e, so there's no need to port those values. Were this an NPC with a character sheet, you might give the character proficiency in the highest two save types, but otherwise there's no direct correlation between the old saving throws and the new ones.
Special attacks and moves are ported over to 5e almost directly, with adjustments for DC saves as described in the conversion document (10 plus the relevant ability score).
The result of this port is a great example of how a monster can translate smoothly from an old edition to the latest. All of the defining traits are present, the ability scores are exactly the same and the stats feel very much the same, and it has scaled down just a little to account for the shift in challenge rating bias.
The process can be more complex, of course. For very specific attacks or spells that simply don't have a direct equivalent, adaptation must be done. Ultimately, though, Wizards of the Coast has done most of the work for you; refer to their tables defining how much damage a monster of a specific size, type, and challenge rating should deal, and then structure the abilities or spells around that information. In the end, you'll have a well ported monster that accurately reflects its ancient origins.