Upgrading monsters and their CR

[Magitech Knight]

How would a GM go about upgrading a monster and its Challenge Rating? Say a mad necromancer dips a skeleton in some sort of liquid metal that makes them stronger and harder to break. And or gives the skeleton a magic weapon such as a vorpal blade or a +3 weapon or armor. I did say he was MAD right?

How do you go about calculating the new CR for this creature?

Also are there any examples of necrotechnology in Pathfinder or D&D modules?

[Esper Ranger]

I too am curious about this.

[ShadowcatX]

There are no examples of necrotechnology in pathfinder AFAIK. You may consider looking into Iron Kingdoms, they did some of it. And of course there are scores of steam punk books for 3.5, several of which might also give you what you seek. Pathfinder steampunk / magitek books would be awesome (hint hint 3pp).

Now as the CR, eyeball it. Or better yet, take an appropriate monster for their CR and just reskin it. (Ie. Change the way it looks to be what you want it to be.)

[Magitech Knight]

I'm not entirely sure how to do that. Does a skeleton with a plus 3 sword become a CR 3 monster? If I throw in plus 2 armor does it become a CR 5 monster?

[Mike J]

You use the tables found on Bestiary pg 291, Bestiary II pg 293, Bestiary III pg 291 (the last two go to CR 25!!!).

Look up the base monster and see how it compares on the table. It might be over on hit points, low on AC, middle on attack, etc. Apply the changes to your monster (raise AC, whatever) and look up the new CR, keeping in mind how it varied from the table. As a double-check, compare the new monster to a similar monster with the same CR.

As to your specific questions, a base skeleton (CR 1/3) with a +3 sword and no other adjustments is probably still CR 1/3 or maybe CR 1/2 (one CR step higher than the base creature) because only two stats have changed - attack and damage. Everything else is still the same. If you added +2 natural armor as well as the +3 sword, I'd still only rate it as CR 1 because hit points and saves didn't change. This is from finding a "best fit" row in the table.

[clff rice]

Well the advanced template could represent an augmented skeleton. As for a magical weapon.

A Skeleton WIth a +3 sword would have the attack of a Cr 2 critter the damage of a Cr 1 critter and the hp of a regular cr 1/3 critter.

So standard Skeleton with +3 sword is around cr 1 or so.

An advanced skeleton with a +3 sword would be around cr 2.

Now you have the problem of the players getting a +3 sword right off the bat for a still relatively easy encounter. 18,000 gold pieces. + whatever the masterwork cost is for the weapon.