The Lightspawn card gets damage equal to it's health. Could it theoretically one hit a hero(30 damage)? What is needed to do so?
What is the most damage it can do "practically" (when the moons aren't aligned, using mainly basic cards, consistently, etc).
A Priest using Power word : shield and Divine spirit twice each can get the Lightspawn to 36 Attack ( (5 + 2 + 2) * 4 = 36). So it could kill a hero in 1 hit. It can also all be played on the same turn has the Lightspawn (4 mana for the Lightspawn, 1 per Power Word: Shield and 2 per Divine spirit, 10 mana).
Using other cards that boost health like Shattered Sun Cleric, Defender of Argus or Temple enforcer you could get it even higher but that requires more turns.
There's other cards to boost health that could help gain even more health.
The card that helps the most is Divine spirit, doubling the health and attack of a Lightspawn.
So playing all those and being able to keep the Lightspawn full of health, you could get a 6 (2 enforcers) + 2 (2 Shattered Sun cleric) + 2 (2 Defender of Argus) + 4 (2 Power word: shield) = 14 + 5(base health of Lightspawn) = 19 * 4 (Divine spirit twice) = 76 attack/health minion.
From a realistic basis, you can usually get at least a Divine Spirit/Power Word Shield combo which would give you a 14/14 pretty often (make sure you shield first). Although most people won't just leave a Lightspawn as is. It will usually get silenced or removed (hexed, polymorphed, straight up killed), but if they leave it alone, you can get in pretty good shape.
For a theoretical, Jonathan is close but he's missing a card, the Dark Cultist.
Base 5 health + 6 from Temple Enforcers, 2 from Defenders of Argus, 2 from Sun Cleric, and 4 from Power Word Shield gives you a base of 19 to start with.
If you were able to suicide a Dark Cultist (on death +3 health to a friendly minion) and buff the Lightspawn you can get another 6 health. 25 health at this point. This would give you a theoretical maximum of 100.
If you had a Baron Rivendare on the board and the "moons perfectly align" (all 4 deathrattles went to the Lightspawn), you could get another 12 health onto the Lightspawn. Which would give you a 31 * 4 = 124.
This is only limiting priest cards. If you thoughtstole 2 Avenge cards from a Paladin and had them trigger as well, that would give your Lightspawn another 4 health (not sure how the +3 attack factors in because of the Lightspawn's ability).
If you also thoughtstole a sword of justice we can add another point of health.
Final theoretical Priest max base Lightspawn 5 + 6 (Temple Enforcers) + 2 (Sun Cleric) + 2 (Defender of Argus) + 4 (Power word shield) + 12 (Dark Cultist/Baron) = 33 base health. 33 * 4 = 132
4 denoted for double divine spirit
These are assuming perfect thoughtstolen cards.
Max vs Paladin: +1 (Sword of Justice) +4 Avenge = 38 * 4 = 152
Max vs Warrior: -1 to Activate Rampage +6 Rampage = 38 * 4 = 152
Max vs Warlock: +8 (2x Power Overwhelming) = 41 * 4 = 164 (Dies at end of turn)
Max vs Druid: +4 (2x Mark of Wild) +8 (2x Mark of Nature) = 45 * 4 = 180
That would take several turns to setup and perfect alignment of randomness.
I'd like to add a similar alternative. Using Mogu'shan Warden (1/7 Taunt), Power Word: Shield (+2 Health) and Divine spirit (double Health) twice and then Inner Fire (makes a Minion's Attack equal to its Health), you get a 44/44 Minion. The good part about this, is that if the Minion is dealt a lot of damage, it doesn't lose any damage.
There is also Temple Enforcer (6 mana, gives target friendly Minion + 3 Health). This easily works in conjunction as well.
