The highest possible damage for Armageddon is 14,905 points per unit. This requires a level 108 hero with Expert Fire Magic, Expert Sorcery, Orb of Tempestuous Fire, maximum Power (99) and Sorcery as the hero's specialty (bonus 5% to Sorcery per level). The highest possible amount of unit stacks in battle is 20 per side + 3 War Machines per side. Therefore, the maximum possible damage to deal to all units on the battlefield, provided no spell immunity or resistance is in play, is 14,905 * 46 = 685,930 points of damage. However, if all creatures on the battlefield are Water Elementals, which are vulnerable to fire, highest possible damage is 14,905 * 2 * 40 + 14,905 * 6 = 1,281,830. However, because the maximum possible Power is 99, the most Water Elementals you can summon in a single stack is 396, which only has 11,880 health, which is 3,025 less than the 14,905 damage each stack takes (and that 14,905 is before the Water Elementals' "double damage" effect). This means that the most damage Armageddon can do in practice is 14 stacks of at least 994 Water Elementals + 26 stacks of 396 summoned Earth Elementals (as 396 summoned Water Elementals don't have enough health to take even 14,905 damage, let alone twice that much) + 1 Ballista (250 health), 2 Ammo Carts (100 health each) and First Aid Tents (75 health each), and a Catapult (1,000 health) = 14 * 2 * 14,905 + 26 * 14,905 + 250 + 200 + 150 + 1000 = 806,470 damage. As a comparison, Implosion's maximum damage is 22,711.
all the spell pages are still using the old https://heroes.thelazy.net/index.php?title=Template:Spell_probability_table&direction=prev&oldid=36155 probability system, which was borrowed from 'tribute to strategists' guide. 20% for armageddon in the dungeon's guild means a chance to get the spell in a brand new 4th level guild, considering there are no banned 4th level spells on the map: 2 slots with 10% probability in each one.
this tab was pretty good in its days, but not so in times of banned spells and spell research.
first, on hota maps without water armageddon's probability is not 10 out of 100, but 10 out of 100 minus Water Walk chance equals 90. if for some reason we have banned Town Portal, its turning to 10 out of 80, and so on.
second, its useful neither for hota's spell research, nor for hd+'s guild rebuilding probability calculations.