You can look this up in the spell text names, you can look this up in the spell texture names, or you can just look in GameMechanics.
Open the game folder, find the data folder, and there's a ZIP archive called data. Go inside and I'll show the path to them in screenshots:
I remembered that I had already listed them out for creating duel heroes. Here you go:
Spells
1-2 LIGHT
<Item>SPELL_BLESS</Item>
<Item>SPELL_HASTE</Item>
<Item>SPELL_STONESKIN</Item>
<Item>SPELL_DISPEL</Item>
<Item>SPELL_REGENERATION</Item>
3-5
<Item>SPELL_BLOODLUST</Item>
<Item>SPELL_DEFLECT_ARROWS</Item>
<Item>SPELL_ANTI_MAGIC</Item>
<Item>SPELL_TELEPORT</Item>
<Item>SPELL_DIVINE_VENGEANCE</Item>
<Item>SPELL_HOLY_WORD</Item>
<Item>SPELL_RESURRECT</Item>
1-2 Dark
<Item>SPELL_CURSE</Item>
<Item>SPELL_SLOW</Item>
<Item>SPELL_SORROW</Item>
<Item>SPELL_DISRUPTING_RAY</Item>
<Item>SPELL_PLAGUE</Item>
3-5
<Item>SPELL_WEAKNESS</Item>
<Item>SPELL_FORGETFULNESS</Item>
<Item>SPELL_BERSERK</Item>
<Item>SPELL_BLIND</Item>
<Item>SPELL_HYPNOTIZE</Item>
<Item>SPELL_UNHOLY_WORLD</Item>
<Item>SPELL_VAMPIRISM</Item>
1-2 Destructive
<Item>SPELL_MAGIC_ARROW</Item>
<Item>SPELL_STONE_SPIKES</Item>
<Item>SPELL_LIGHTNING_BOLT</Item>
<Item>SPELL_ICE_BOLT</Item>
3-5
<Item>SPELL_FIRE_BALL</Item>
<Item>SPELL_FROST_RING</Item>
<Item>SPELL_FIRE_WALL</Item>
<Item>SPELL_CHAIN_LIGHTNING</Item>
<Item>SPELL_METEOR_SHOWER</Item>
<Item>SPELL_IMPLOSION</Item>
<Item>SPELL_ARMAGEDDON</Item>
<Item>SPELL_DEEP_FREEZE</Item>
1-2 Summon
<Item>SPELL_MAGIC_FIST</Item>
<Item>SPELL_LAND_MINE</Item>
<Item>SPELL_WASP_SWARM</Item>
<Item>SPELL_ANIMATE_DEAD</Item>
<Item>SPELL_ARCANE_CRYSTAL</Item>
3-5
<Item>SPELL_PHANTOM</Item>
<Item>SPELL_EARTH_QUAKE</Item>
<Item>SPELL_BLADE_BARRIER</Item>
<Item>SPELL_SUMMONE_ELEMENTALS</Item>
<Item>SPELL_SUMMONE_HIVE</Item>
<Item>SPELL_CELESTIAL_SHIELD</Item>
<Item>SPELL_CONJURE_PHOENIX</Item>
Dorg
