One of my player just reached Magus 7 / Soul Drinker 2, gaining the following ability:
Energy Drain (Su): At 2nd level, a souldrinker gains the
energy drain ability. By making a touch attack as a standard
action, she bestows one negative level on the target (two levels
on a critical hit). The souldrinker gains 5 temporary hit
points for each negative level she bestows on an opponent.
These temporary hit points last for a maximum of 1 hour.
The DC to remove this negative level is 10 + the souldrinker’s
class level + her spellcaster ability modifier. At 6th level, this
touch attack bestows two negative levels (four on a critical hit).
How would the energy drain fit in with his spell combat, etc - it seems to more than lean toward OP if this can apply at the same time as spell delivery and a couple of melee attacks. And secondly, given that the text mentions no opportunity to save against this, is it a guaranteed negative level on every touch attack?
By RAW, it doesn't fit into spell combat. Supernatural abilities aren't spells. And you can't make the substitution of a spell cast for a supernatural ability.
If the GM house rules and allows the substitution, I tend to think its power is still largely curtailed by the fact that the magus probably has something more potent he be could be discharging.
I think the energy drain ability is balanced by its poor action economy. Inflicting a negative level as a standard action is not spectacular. A full-attack is usually more powerful. Letting him stack the two together seems too powerful.
I could see him being able to deliver the energy drain through a spell strike, but not in combination with a full attack. Even then, I think I'd make it cost him a Magus Arcana.
How would the energy drain fit in with his spell combat, etc
Spell Combat is a full-round action that lets you make your attacks with a melee weapon and also cast a spell from the magus spell list with a casting time of 1 standard action. Since energy drain is neither a melee weapon attack, nor a spell from the magus spell list, it doesn't apply.
Spellstrike lets you deliver a touch spell you cast from the magus spell list through your weapon. As energy drain is not a spell cast from the magus spell list, it doesn't apply.
+1 arcana to spellstrike/spell combat (Su)? It doesn't seem to be particularly overpowered and it makes sense thematically. It should definitely cost an Arcana to do so, though. And he uses it in place of a spell for this purpose. So. But honestly don't listen to me too much, I tend not to have any idea what I'm talking about. Just my two coppers.
I think the conductive weapon property could do it. Although conductive normally requires two uses of your supernatural ability, Energy Drain for the souldrinker is unlimited (it kind of IS what the prestige class is all about).
This is not RAW, and would be for houseruling:
Flavour wise, though, it's quite black bladeish. I'd allow it, as part of spell combat. Even as a full attack, It's only one of the attacks.
And, of course, it's no longer a touch attack...
I think the conductive weapon property could do it. Although conductive normally requires two uses of your supernatural ability, Energy Drain for the souldrinker is unlimited
Conductive: "A conductive weapon is able to channel the energy of a spell-like or supernatural ability that relies on a melee or ranged touch attack to hit its target (such as from a cleric's domain granted power, sorcerer's bloodline power, oracle's mystery revelation, or wizard's arcane school power). When the wielder makes a successful attack of the appropriate type, he may choose to expend two uses of his magical ability to channel it through the weapon to the struck opponent, who takes the effects of the weapon attack and the special ability. (If the wielder has unlimited uses of a special ability, he may channel through the weapon every round.) For example, a paladin who strikes an undead opponent with her conductive greatsword can expend two uses of lay on hands ability (a supernatural melee touch attack) to deal greatsword damage and damage from one use of her lay on hands. This weapon property can only be used once per round, and only works with magical abilities of the same type as the weapon (melee or ranged)."