If you don't chose the undead as a ranger favored enemy then you will get the +1 for all bonuses against it as an undead slayer, but they will not improve over time. On the other side of things. If you take it as a ranger, you will not get a +1 (ranger) +1 ( undead slayer) to the bonus vs undeads but the one for the ranger will go up 1 points each five levels you take as a ranger.
but what if you don't chose undead as ranger and become an undead slayer? Would the bonus stack or just be +1 against undead?
`DebugMode 1`dm_givelevel 39*click on your character*