Void Seeker

If a spell or ability instructs a particular object to deal damage, including the card the ability is on, then that object is dealing the damage, not the spell or ability. So Unyielding Spirit does stop damage from Void Seeker, Stormbringer, or Yasuo, Remorseless, but does not stop damage from Challenge, Last Breath, or Carnivorous Snapvine.

This includes "direct kill" spells you control like Hidden Blade. It also includes damage spells like Void Seeker, provided they mark lethal damage on the unit that immediately causes it to die. In all cases, the unit has to actually die-if that death is replaced by a card like Unlicensed Armory, then you didn't kill a unit with a spell.

135.2.b.5.a. The complement of a game action is made up of the players or Game Objects that perform that game action, that that game action is performed on, the condition under which it is performed, and the duration for its effect to last. Some game actions omit a complement entirely. Example: Void Seeker is made up of two instructions. The first is "deal 4 to a unit at a battlefield." Here "deal 4" is the game action, while "a unit at a battlefield," is its complement. The second instruction, "draw 1," has no complement. Example: Teemo, Strategist's defend trigger reads "When I defend, choose an enemy unit here and reveal the top 5 cards of your Main Deck. Deal 1 to that unit for each card with Hidden revealed this way, then recycle the revealed cards." There are four instructions here: "choose an enemy unit here," "reveal the top 5 cards of your Main Deck," "deal 1 to that unit for each card with Hidden revealed this way," and "recycle the revealed cards."

138.1. Flavor text is text that appears at the bottom of some cards to provide aesthetic enjoyment. It conveys no gameplay information. Example: The card Void Seeker features a quote from the character Kai'Sa. It does not have the Kai'Sa tag and has no gameplay connection to the Champion unit Kai'Sa.

359.3.e.5. If any of the spell's targets are no longer legal, those game objects, players, or zones are unaffected by the spell as it resolves. Any instructions related to an illegal target can't be followed. Example: A player plays Void Seeker, a spell that says "Deal 4 to a unit at a battlefield. Draw 1." The unit's controller uses a Reaction to move the unit to their base. Since the unit is no longer a legal target, it is not dealt any damage. Void Seeker's controller still draws 1. Example: A player plays Bellow's Breath targeting a unit in combat that reads in part "I can't be chosen by enemy spells and abilities unless I'm in combat." In reaction, that unit's controller plays Flash, moving the unit to their base. The unit is no longer in combat, so it is no longer a legal target for Bellow's Breath. The unit will be unaffected by Bellow's Breath as it resolves. Example: A player plays Hidden Blade from the facedown zone at a battlefield, targeting an enemy unit. In reaction to Hidden Blade, their opponent plays Tideturner from facedown at another battlefield, choosing to swap locations with the unit Hidden Blade targeted. When Hidden Blade resolves, the chosen unit is no longer at the appropriate battlefield, so any instructions related to that unit are ignored.

417.1.e.1. Only Valid Damage is Dealt. Example: A unit reads "when I take damage, give me +2 [M] this turn." A spell is played that Prevents the next 3 damage the unit would take. If a player plays Hextech Ray targeting the unit, it will take no damage and its triggered ability will not trigger. If that player had played Void Seeker instead, it would be Dealt 1 and trigger its ability.

417.6.a. If a game effect does not specify a source, the game effect describing the Deal action is the source. Example: Void Seeker is a spell that reads "Deal 4 to a unit at a battlefield. Draw 1." The damage that Void Seeker instructs you to deal is dealt by Void Seeker.