Maybe?

nebulo/src/main/java/dev/louis/nebulo/spell/CloudJumpSpell.java

@@ -2,14 +2,15 @@
 
 import dev.louis.nebula.api.spell.Spell;
 import dev.louis.nebula.api.spell.SpellType;
+import net.minecraft.entity.player.PlayerEntity;
 import net.minecraft.particle.ParticleTypes;
 import net.minecraft.server.network.ServerPlayerEntity;
 import net.minecraft.sound.SoundCategory;
 import net.minecraft.sound.SoundEvents;
 
 public class CloudJumpSpell extends Spell {
-    public CloudJumpSpell(SpellType<?> spellType) {
-        super(spellType);
+    public CloudJumpSpell(SpellType<?> spellType, PlayerEntity player) {
+        super(spellType, player);
     }
 
     @Override

src/main/java/dev/louis/nebula/Nebula.java

@@ -3,6 +3,7 @@
 import com.mojang.logging.LogUtils;
 import dev.louis.nebula.api.spell.SpellType;
 import dev.louis.nebula.command.NebulaCommand;
+import dev.louis.nebula.manager.NebulaManager;
 import dev.louis.nebula.networking.SpellCastC2SPacket;
 import net.fabricmc.api.ModInitializer;
 import net.fabricmc.fabric.api.networking.v1.ServerPlayNetworking;

src/main/java/dev/louis/nebula/api/manager/mana/ManaManager.java

@@ -47,8 +47,6 @@ public interface ManaManager {
      */
     boolean hasEnoughMana(int mana);
 
-    //Thinking about removal as it would isolate the ManaManager from anything Spell related :)
-    @Deprecated
     /**
      * @param spellType The SpellType which should be checked.
      * @return If enough mana is available for the specified SpellType.

src/main/java/dev/louis/nebula/api/spell/Spell.java

@@ -2,7 +2,6 @@
 
 import dev.louis.nebula.api.manager.spell.SpellManager;
 import net.minecraft.entity.player.PlayerEntity;
-import net.minecraft.network.PacketByteBuf;
 import net.minecraft.util.Identifier;
 
 /**
@@ -12,16 +11,17 @@
 public abstract class Spell {
     private static final int DEFAULT_SPELL_AGE = 3 * 20;
 
-    private final SpellType<?> spellType;
-    private PlayerEntity caster;
+    protected final SpellType<?> spellType;
+    protected final PlayerEntity caster;
 
     protected boolean wasInterrupted;
     protected boolean stopped;
 
     public int age = 0;
 
-    public Spell(SpellType<?> spellType) {
+    public Spell(SpellType<?> spellType, PlayerEntity caster) {
         this.spellType = spellType;
+        this.caster = caster;
     }
 
     /**
@@ -53,10 +53,6 @@ public SpellType<? extends Spell> getType() {
         return this.spellType;
     }
 
-    public int getAge() {
-        return this.age;
-    }
-
     public int getDuration() {
         return DEFAULT_SPELL_AGE;
     }
@@ -69,10 +65,6 @@ public int getDuration() {
     public void finish() {
     }
 
-    public void setCaster(PlayerEntity caster) {
-        this.caster = caster;
-    }
-
     /**
      * Used to check if the spell should be stopped. <br>
      * It is important to check super or check if the spell was interrupted {@link Spell#wasInterrupted()}. <br>
@@ -98,8 +90,6 @@ public final void interrupt() {
      * <br>
      * Unlike {@link Spell#interrupt()} if you call this method it is expected that the spell has finished execution.<br>
      */
-    //Thinking about possible change.
-    @Deprecated
     protected final void stop() {
         this.stopped = true;
     }
@@ -121,16 +111,6 @@ public boolean wasInterrupted() {
         return this.wasInterrupted;
     }
 
-    /**
-     * Write additional casting data about the spell to the buf.
-     * @param buf The buf to be written to.
-     * @return The buf after being written to.
-     */
-    public PacketByteBuf writeBuf(PacketByteBuf buf) {
-        return buf;
-    }
-
-
     @Override
     public String toString() {
         return this.getClass().getSimpleName() +

src/main/java/dev/louis/nebula/api/spell/SpellType.java

@@ -68,8 +68,8 @@ public int getManaCost() {
         return manaCost;
     }
 
-    public T create() {
-        return this.spellFactory.create(this);
+    public T create(PlayerEntity caster) {
+        return this.spellFactory.create(this, caster);
     }
 
     @Override
@@ -134,7 +134,8 @@ public SpellType<T> build() {
 
     @FunctionalInterface
     public interface SpellFactory<T extends Spell> {
-        T create(SpellType<T> spellType);
+
+        T create(SpellType<T> spellType, PlayerEntity caster);
     }
 
     @FunctionalInterface

src/main/java/dev/louis/nebula/NebulaManager.java → src/main/java/dev/louis/nebula/manager/NebulaManager.java

@@ -1,5 +1,6 @@
-package dev.louis.nebula;
+package dev.louis.nebula.manager;
 
+import dev.louis.nebula.Nebula;
 import dev.louis.nebula.api.manager.mana.ManaManager;
 import dev.louis.nebula.api.manager.mana.entrypoint.RegisterManaManagerEntrypoint;
 import dev.louis.nebula.api.manager.mana.registerable.ManaManagerRegistrableView;

src/main/java/dev/louis/nebula/manager/spell/NebulaSpellManager.java

@@ -73,18 +73,9 @@ public boolean forgetSpell(SpellType<?> spellType) {
         return shouldSync;
     }
 
-    /**
-     * This Method does not return an unmodifiable set.
-     * So that it could theoretically be modified.
-     */
-    protected Set<SpellType<?>> getCastableSpells() {
-        return learnedSpells;
-    }
-
     @Override
     public boolean cast(SpellType<?> spellType) {
-        var spell = spellType.create();
-        spell.setCaster(this.player);
+        var spell = spellType.create(this.player);
         return this.cast(spell);
     }
 
@@ -97,9 +88,9 @@ public boolean cast(Spell spell) {
             if (this.isServer()) {
                 spell.applyCost();
                 spell.cast();
-                this.activeSpells.add(spell);
+                if (spell.getDuration() > 0) this.activeSpells.add(spell);
             } else {
-                ClientPlayNetworking.send(new SpellCastC2SPacket(spell));
+                ClientPlayNetworking.send(new SpellCastC2SPacket(spell.getType()));
             }
             return true;
         }
@@ -171,9 +162,9 @@ public static boolean receiveSync(UpdateSpellCastabilityS2CPacket packet, Client
     @Override
     public void writeNbt(NbtCompound nbt) {
         NbtList nbtList = new NbtList();
-        for (SpellType<?> spell : getCastableSpells()) {
+        for (SpellType<?> spellType : this.getLearnedSpells()) {
             NbtCompound nbtCompound = new NbtCompound();
-            nbtCompound.putString(SPELL_NBT_KEY, spell.getId().toString());
+            nbtCompound.putString(SPELL_NBT_KEY, spellType.getId().toString());
 
             nbtList.add(nbtCompound);
         }

src/main/java/dev/louis/nebula/mixin/PlayerMixin.java

@@ -1,9 +1,9 @@
 package dev.louis.nebula.mixin;
 
-import dev.louis.nebula.NebulaManager;
 import dev.louis.nebula.api.NebulaPlayer;
 import dev.louis.nebula.api.manager.mana.ManaManager;
 import dev.louis.nebula.api.manager.spell.SpellManager;
+import dev.louis.nebula.manager.NebulaManager;
 import net.minecraft.entity.EntityType;
 import net.minecraft.entity.LivingEntity;
 import net.minecraft.entity.damage.DamageSource;

src/main/java/dev/louis/nebula/networking/SpellCastC2SPacket.java

@@ -1,7 +1,6 @@
 package dev.louis.nebula.networking;
 
 import dev.louis.nebula.Nebula;
-import dev.louis.nebula.api.spell.Spell;
 import dev.louis.nebula.api.spell.SpellType;
 import net.fabricmc.fabric.api.networking.v1.FabricPacket;
 import net.fabricmc.fabric.api.networking.v1.PacketSender;
@@ -13,19 +12,23 @@
 /**
  * Not a FabricPacket because of Restriction in the api.
  */
-public record SpellCastC2SPacket(Spell spell) implements FabricPacket {
-    public static final Identifier ID = new Identifier(Nebula.MOD_ID, "spellcast");
+public class SpellCastC2SPacket implements FabricPacket {
+    private static final Identifier ID = new Identifier(Nebula.MOD_ID, "spellcast");
     public static final PacketType<SpellCastC2SPacket> TYPE = PacketType.create(ID, SpellCastC2SPacket::read);
+    public final SpellType<?> spellType;
+
+    public SpellCastC2SPacket(SpellType<?> spellType) {
+        this.spellType = spellType;
+    }
 
     public static SpellCastC2SPacket read(PacketByteBuf buf) {
         SpellType<?> spellType = buf.readRegistryValue(SpellType.REGISTRY);
-        if(spellType == null) throw new IllegalStateException("Spell type not found in registry");
-        Spell spell = spellType.create();
-        return new SpellCastC2SPacket(spell);
+        if (spellType == null) throw new IllegalStateException("Spell type not found in registry");
+        return new SpellCastC2SPacket(spellType);
     }
 
     public void write(PacketByteBuf buf) {
-        buf.writeRegistryValue(SpellType.REGISTRY, spell.getType());
+        buf.writeRegistryValue(SpellType.REGISTRY, spellType);
     }
 
     @Override
@@ -34,10 +37,13 @@ public PacketType<?> getType() {
     }
 
     public static void receive(SpellCastC2SPacket packet, ServerPlayerEntity player, PacketSender responseSender) {
-        var spell = packet.spell();
-        spell.setCaster(player);
-        player.server.executeSync(() -> {
-            player.getSpellManager().cast(spell);
-        });
+        player.server.executeSync(() -> player.getSpellManager().cast(packet.spellType));
+    }
+
+    @Override
+    public String toString() {
+        return "SpellCastC2SPacket[" +
+                "spellType=" + spellType + ']';
     }
+
 }