You would simply say “The spellcasting ability modifier for this spell is Wisdom.”
Wisdom is the stat that represents your willpower, your experience, and your ability to perceive the world around you. If something attacks your mind, it is most often resisted with Wisdom for this reason.
Realistically, it probably shouldn’t be a spell, and it definitely shouldn’t be this complicated. Spells used to have this level of granularity in earlier editions, and 5e specifically moved away from that for clarity and speed of play.
My recommendation is to decide if the person this item was created for (not necessarily the PC using it) is supposed to die or not when using it. If they are, then the item just kills them. If not, they fall unconscious at 0 HP, then suffer one failed death save as normal when the item detonates. Don’t mess about with charging it with death saves or exhaustion levels, just have it do some damage.