The line to pay attention to is "Deathless armor does not ... protect against ... negative energy effects that do not deal damage..."
A Shadow's negative energy attack does not deal hit point damage; it deals ability damage.
Think of Deathless as being akin to Resist Energy, except it's versus negative energy instead of elemental energy.
It's buried in the APG Eidolon description. Many people are unaware of it:
APG Summoner wrote:
magic items interfere with the summoner's connection to his eidolon. As a result, the summoner and his eidolon share magic item slots. For example, if the summoner is wearing a ring, his eidolon can wear no more than one ring. In case of a conflict, the items worn by the summoner remain active, and those used by the eidolon become dormant. The eidolon must possess the appropriate appendages to utilize a magic item.
I am unaware if the Unchained Summoner removed this limitation.
The Unchained errata has no affect on his Summoner. It was played at level 2, and is not required to be updated. It sounds like the OP simply doesn't want to play as a Summoner.
I would hesitate to use the word "Gimp". Some people purely cannot accomplish the same build with an Unchained Summoner as they could with an APG Summoner. I use myself as an example. My goal was to build a Summoner as a "Necromancer", complete with a Greatsword-wielding Skeletal Champion as my personal undead thrall, and specialize in summoning Undead via the Skeletal Summoner feat.
That build is not possible with an Unchained Summoner (barring future archetypes or updates). I've read other examples that are similarly no longer viable as well.
Julius Foxton wrote:
but them's the rules ;)
Them is not.
I've provided the relevant quotes.
Each of those variants, independently, cost double.
Mathematically, applying both would mean tripling.
Or even quadrupling, but it's such a sub par tactic already I wouldn't want to suffer that logic upon anyone.
Plus, you still have to get this passed by your GM.
For ease of reference going forward:
Animate Dead wrote:
Edit: edited out some of the non HD-related content to make it more readable.
Numerous variant skeletons exist, such as those whose bones burn with an unending fire and those who drip with gore and reassemble themselves over time. Both of these variant skeletons can be created using animate dead, but they count as twice their normal number of Hit Dice per casting.
I presume you're talking about "doubling HD" for purposes of Animate Dead?
In that case, I wouldn't count it as just double, either. Probably triple, as otherwise you're getting the second template basically "for free".
Creating a Bloody Skeleton shouldn't cost the same as creating a Bloody Burning Skeleton.