Вампирский меч, сделал как пример. Излечивает носителя на половину от…

Content.Server/Weapons/Melee/Vampire/VampireHealComponent.cs

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+using Robust.Shared.GameObjects;
+using Robust.Shared.Serialization.TypeSerializers.Implementations.Custom;
+using Content.Shared.FixedPoint;
+
+namespace Content.Server.Weapons.Melee.Vampire;
+
+[RegisterComponent]
+public sealed partial class VampireHealComponent : Component
+{
+    [DataField("healMultiplier")]
+    public FixedPoint2 HealMultiplier = 0.5f;
+}

Content.Server/Weapons/Melee/Vampire/VampireHealSystem.cs

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+using Content.Shared.Damage;
+using Content.Shared.Damage.Events;
+using Content.Shared.Damage.Components;
+using Content.Shared.Hands.EntitySystems;
+using Content.Shared.Mobs.Components;
+using Content.Shared.Mobs;
+using Content.Server.Damage.Systems;
+using Content.Shared.FixedPoint;
+using Robust.Shared.GameObjects;
+
+namespace Content.Server.Weapons.Melee.Vampire;
+
+public sealed class VampireHealSystem : EntitySystem
+{
+    [Dependency] private readonly DamageableSystem _damageable = default!;
+    [Dependency] private readonly SharedHandsSystem _hands = default!;
+
+    public override void Initialize()
+    {
+
+        SubscribeLocalEvent<DamageableComponent, DamageChangedEvent>(OnDamageChanged);
+    }
+
+    private void OnDamageChanged(EntityUid target, DamageableComponent comp, ref DamageChangedEvent args)
+    {
+
+        if (args.DamageDelta == null)
+            return;
+
+
+        if (args.Origin == null)
+            return;
+
+        var attacker = args.Origin.Value;
+
+
+        var totalDamage = args.DamageDelta.GetTotal();
+        if (totalDamage <= FixedPoint2.Zero)
+            return;
+
+
+        if (attacker == target)
+            return;
+
+
+        if (!TryComp<MobStateComponent>(target, out var mobState))
+            return;
+
+
+        if (mobState.CurrentState == MobState.Dead)
+            return;
+
+
+        if (!_hands.TryGetActiveItem(attacker, out var weapon))
+            return;
+
+
+        if (!TryComp<VampireHealComponent>(weapon.Value, out var vampire))
+            return;
+
+
+
+        var healAmount = totalDamage * vampire.HealMultiplier;
+
+        if (healAmount <= FixedPoint2.Zero)
+            return;
+
+
+
+        if (!TryComp<DamageableComponent>(attacker, out var attackerDamage))
+            return;
+
+        var healSpecifier = new DamageSpecifier();
+
+        foreach (var (type, amount) in attackerDamage.Damage.DamageDict)
+        {
+            if (amount > FixedPoint2.Zero)
+            {
+                healSpecifier.DamageDict[type] = -healAmount;
+            }
+        }
+
+        if (healSpecifier.DamageDict.Count == 0)
+            return;
+
+        _damageable.TryChangeDamage(attacker, healSpecifier, origin: weapon.Value);
+    }
+}

Resources/Prototypes/Entities/Objects/Weapons/Melee/vampire_sword.yml

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+- type: entity
+  id: VampireSword
+  parent: BaseItem
+  name: vampire sword
+  description: It thirsts for blood.
+  components:
+  - type: Sprite
+    sprite: Objects/Weapons/Melee/captain_sabre.rsi
+    state: icon
+  - type: MeleeWeapon
+    damage:
+      types:
+        Slash: 15
+  - type: VampireHeal
+    healMultiplier: 0.5