still should do the test on whether the +x phys / +x magic damage is adding to base damage and scaled by % ed like bind demon is
its just so tiny in comparison I never bothered but a pure summoner could care. well not really still just tiny
also I have no idea, does it teleport non-demon / non-mercenary summons, like golems or wolves? I doubt it, haven't tested. Definitely doesn't teleport merc
its very useful when fighting something like a P8 TZ96 tier 5 herald and its minions to teleport just a short distance away and deathmark your demon in front of the pack, so neither you nor your merc get any aggro. Then you can just spam them to death without taking any damage. So its really an upside it doesn't teleport your merc, at least in pvm. In PvP I haven't gotten it to actually hit anyone because it can just set up a long range blood boil that kills them instantly
Havnt tested either, but I’m pretty sure it should:
Teleport only demons, as other skills like mastery and blood oath only affects demons, so all the warlock skills are probably tagged as demon pet type only.
And the phys dmg part of death mark should act like any other +dmg in the game, applied before it gets multiplied. Not huge, but it could make a small difference with enough +skills as you increase both the flat dmg and the ed%.