As a mage, if you are taking damage you have a problem. Alchemists have golems to fix this problem, while Archmages have movement speed gear and buffs. Necromancers have disposable pets and resistance, defense, and armor upgrades, most of which improve at below 0 hp. The problem is, even with gear and a high-Constitution race, the "below hp" regime is generally less than half of their hp. Playing with a "the first 2/3 of my hp are disposable" mentality is liable to give you a dead character.

Enter the Hiemal shield. As an always-on sustain that substantially improves a necromancer's hit points, the shield makes incidents like "Oops, that wyrmic can do 300 damage with a breath attack" into embarrassing mishaps instead of new character generation.

“Sounds great. Why is it love/hate?"

Love, because the Hiemal shield is iconic - it is recognizably and unquestionably a necromancer talent.

Hate, because… try benchmarking shields. AMs get 2000+ hp shields. The Disruption shield turns your mana into hp, and the Temporal shield heals you. The Hiemal shield is about 1/4 of that, and it just unsustains. You can damage nearby monsters when it happens, but that doesn't help you survive in nearly as direct a fashion as healing.

And the cost of the Hiemal shield is absolutely not cheap. This thing eats souls. Hiemal shield takes 1 soul and 30 mana to sustain — no problem — but eats another soul if it takes even 1 hp of damage and has to refresh. Slip on a rock? That’ll be 1 soul please. Unlike the Stormshield rune, which lets damage below a threshold through and only takes big hits for you, the Hiemal shield takes damage from any attack. Not only that, but when you combine other shields with it — like shield runes — it STILL counts as being damaged and eats a soul afterwards.