Skeletons

You are correct. Resurrection is in a very early stage. In R18 we are adding/changing a few things - like magic attacks being able to influence resurrection chance, resurrection chances, etc.

From the R16 release notes:

• Undead Resurrection Behavior: Undead creatures will now appear to die, only to rise again (they are undead after all). For now they will do this at all times, but in further releases this behavior will be slightly randomized and will mostly only be triggered when Undead are killed by physical means (i.e. magic will usually destroy them the first time). We will also use this behavior as the basis for the Phoenix behavior (we hope to get the Phoenix in R17). This behavior we affectionately call the “I’m getting better!” behavior.