Issues abound with this card, and it is the reason why I consider Valkyrie to be clunky and underpowered.
There is currently no ruling from FFG about how cards like this count for deckbuilding: official word being "build however you like" (per Caleb's answer to Vision's identical situation with mass form). So it could be that her minimum deck size is actually 39 plus this card, or her maximum deck size is actually 51.
Valkyrie can only attach Death-Glow using her hero action Death Perception, where it stays until the enemy is defeated or until returned using Brunnhilde's action "Not this day." Death-Glow is the only way to trigger Flight of the Valkyrior (2 copies) and Shieldmaiden (2 copies) and Valhalla, plus Valkyrie gets some conditional bonuses from Dragonfang and Valkyrie's Spear, and Have at Thee! (3 copies) gains overkill (FAQed to work as intended). Note that in order to trigger Valhalla or Death-Glow's ready effect then Valkyrie herself must be the one to defeat the enemy, not an ally or a support or another hero.
A nasty niche case exists where Death-Glow is attached to an enemy that leaves play without being defeated, at which point the rules state that the attachment is discarded, and therefore not able to be played again by Death Perception, because it is in the discard pile and not set aside.
Death-Glow is a barrier to entry to 5 of Valkyrie's cards while also locking away some of the value on 5 more of her cards, so if she chooses to ignore it entirely then she is playing with 5 blank and 5 suboptimal cards. Rather than enabling these cards, it is a tax that hampers her flexibility and efficiency especially considering her 5 card hand size and the burden of too many cards costing 3 resources.
It also tries to take her in too many directions at once. Valhalla, Flight of the Valkyrior, "Have at Thee!", and the ready effect only function when Death-Glow is attached to a minion, while Shieldmaiden, Dragonfang, and Valkyrie's Spear all shine when it is attached to the villain. Switching target from the villain back to minions requires going into alter-ego mode and is therefore slow.
This is compounded by both her obligation Trouble in Otherworld and her backbreaking Shadow of the Past, which each decimate what little tempo remains after playing around Death-Glow in the first place.
It could be "fixed" by changing the resource cost to 0, or Death Perception could change from "set-aside Death-Glow upgrade" to "Death-Glow upgrade, wherever it is,"... but in either case it would still be inferior to Nova's nova.