Drain Soul is most importantly an exactly 6 damage spell
In a game that uses cards as non-random but combat as random
That is a big deal (apart from the 12 swing)
Glad to hear that you were impressed by Kumanjaro
It is what I always propose as the "second buy" after Core Set (yet commercially did less well!)
For those who haven't got it, there are crucial pieces of tech in it
Here was an old post explaining why it's so good
Get Conquest
> Enchantment Transfusion (singe-handedly creates action burst combos, protects enchants, targeted counter)
> Wizard's Tower (why we play wizards, 'nuff said)
> Galador (awesome if slightly meta-dependent)
> Guardian Angel (holy gets the best interceptor)
> Surging Wave (range 2 slam 3+ to remove guard markers, extinguish burn, unavoidable vs. gremlins, utility)
> Eagleclaw Boots (wizards use teleport wands, don't like being pushed through thorns, trapped by walls etc)
> Armour Ward / Enchanter's Wardstones (only if playing Mana Denial or Transfusion Curse Stacking)
> Healing Charm (with enchanter's ring, time it right for max. unexpected life extension)
> Gargoyle Sentry (fine interceptor that can emerge from Gate to Voltari and protect it)
> Drain Soul (even a new toy for a Warlock, maybe single toolbox copy to deal exactly 6 damage or a 12 swing)
The 2 variant mages play very different to base set (Jokhtari with a Lair spawning Dire Wolves, Galador, Hunting Bow, Tanglevines, Idol, Deathlock is especially fun in a meta without Iron Golems).
However, what really sold it to me most was the flavour: the whole set is seeped in a Native Shaman vs. Fanatic Missionary colonial fantasy setting that is so refreshingly different. Your posts demonstrate a rich creativity and this setting will appeal to your imagination.