Based on some replies it seems like this card can never happen, but the idea still can be re-used with just a few names changed :P
Card name: Lich King Arthas (undead) Card Rarity: Neutral Legendary Cost: 10 mana crystals
When card is played --- choose one(like druid cards) : Lich King (Heroic) / Arthas (Minion) Heroic as in Lord Jaraxxus
1) Lich King(Heroic):
Kills your hero and replace with the Lich King 15 HP (Might be changed to 12 HP)
Battlecry: Silence all enemy undead minions (such as the cards from Naxxramas and Sylvanas)
Equippe your hero with a 4/5 Frostmourne: restores 1-3 mana crystals upon damaging an enemy.
Hero Power (3 Cost): Frostmourne hungers ---> 50% Freeze, deal 3 damage to a minion, heal for half damage dealt *can be affected by spell power <----- (at first I thought it should be enemy character, but then it would completely overshadow Lord Jaraxxus, which is a class legendary, and should be more powerful than a neutral, and damage to a minion but not enemy minion is to provide situational utility)
2) Arthas(minion): a 7/9 minion (when activated by deathrattle/alarm-o-bot, pulls out Arthas)
Battlecry: Take control of an enemy undead minion.
Freeze any character damaged by this minion.
Deathrattle: Equippe your hero with a 5/4 Frostmourne (Note this is a different Frostmourne)
5/4 Frostmourne: Heals your hero for 2 whenever Frostmourne deals damage (this heal happens after the damage is dealt, unlike Truesilver Champion)
5/4 Frostmourne Deathrattle: Deals 5 damage to your hero.
Thought behind Arthas (minion): For a 10 cost legendary minion, and of course Arthas, I thought it should it should be powerful enough to deal with other high end drops, but should not be able to completely dominate everything, and the Frostmourne Deathrattle is based on what happened to Arthas in Warcraft 3, a side effect of Frostmourne if you will. A silence would lose you some value, but not so much to a point where you just facepalm. And I did think about switching battlecries between Arthas and the Lich King, but then the heroic form would give the player way too much value, even if it is situational. Both battlecries are subjected to change, such as take control of an undead minion for one turn, or summon 2 1/1 skeletons.