Spell Like Abilities as Custom Powers

Am I correct that the “Level #” headings in this section refer to spell level? So that the ability to cast, for instance, Magic Missle once per hour at its normal casting time would cost one custom power under this system? Because that seems awfully generous.
Where I really noticed this was in trying to put together a Mage-Thief class. I thought it might be flavorful to have the class do some of it’s “thiefing” with magic instead of skill. Turns out most of the thief-replacement spells are 1st or 2nd level, so, if I’m interpreting this right, you can directly swap:

  • “Open Locks” for “Knock, 1/hour, Casting Time 10 minutes”,
  • “Find Traps” for “Find Traps (the divine spell), 1/hour, Casting Time 10 minutes”,
  • “Move Silently” for “Inaudibility, 1/hour, Casting Time 10 minutes”,
  • “Climb Walls” for “Spider Climb, at will, Casting Time 10 minutes”,
  • “Hide in Shadows” for “Invisibility, 1/hour, Casting Time 10 minutes”, and
  • “Read Languages” for “Read Languages (the arcane spell), at will, Casting Time 10 minutes”.
    Again, that seems generous.

It’s only really generous in regards to the thief class. Try replacing a fighter’s abilities with magic! More specifically the old school thief class has always been weak and easily replacable by a caster. That’s why they got such a beef up in later editions.

Hmmm. On first glance, the powers you’ve outlined seem like they would make for a fun class, not game-breaking at all. In fact, I’m rather bummed I didn’t think of that class. An elven nightblade with hide in shadows and move silently replaced by at-will invisibility and inaudability 1/hour seems awesome!!
Obviously, the “spell-like” thief would exceed the thief in power at 1st level, with a decrease in relative power each level thereafter, with the “flip” coming at about level 7. The “spell-like” thief’s inability to open multiple locks per hour, and inability to re-hide/re-sneak in short time periods, are definite drawbacks. In my own campaign play, opening locks, sneaking, etc., tend to be “bursty” - “we found the treasure room, quick, let’s get these locks open”.
If you were just intending to run a campaign in early levels, I can see the “mage-thief” being the preferred choice, but there’s no question that “the Prince of Thieves” would be a thief enjoying 1+ proficiency throws with all his thief skills, and not a “mage-thief”.

One of my players is running a nightblade and I’ve started an arcane variant. I’ll see which he prefers.

The Mage-Theif variant might be a neat new take for a Gnome class. Just sayin’.
(Call him a “Spellsneak?”)

Alex: Thanks for taking a look at this. It could be that my experience with the 2nd Ed Player’s Option line has left me overcautious. Your analysis of the Thief vs. “Spell Like” Thief progression has a great “quicker, easier, more seductive” vibe. I’ll let my Atlantean Mentalists keep their spell like abilities. It will be an interesting experiment.