And now for a very specific priest-thief multiclass:
Slayer class
Into every generation, the slayers are born, the chosen ones. They alone have the strength and skill to fight the undead, demons, devils and forces of darkness; to stop the spread of their evil and the swell of their number. They are the slayers.
Weapons: Crossbow, dagger, longsword, shortbow, shortsword, spear, staff
Armor: Leather armor, mithral chainmail
Hit Points: 1d6 per level
Languages. You know Celestial and Diabolic.
Learning Spells. You can permanently learn a priest spell from a spell scroll by studying it for a day and succeeding on a
DC 15 Wisdom check.
Whether you succeed or fail, you expend the spell scroll.
Turn Undead. You know the turn undead spell.
Slaying. If you hit an undead, demon or devil, you deal an extra weapon die of damage. Add additional weapon dice of damage equal to half your level (round down).
Lore of the Order. You have advantage on checks associated with undead, demons and devils.
Deity. Choose a god to serve who matches your alignment (see Deities, Shadowdark pg. 30). You have a holy symbol for your god (it takes up no gear slots).
Spellcasting. You can cast priest spells you know. You do not start play knowing any spells other than turn or gain any additional ones when you gain a level. Instead, you must discover them in play. You otherwise use the rules for casting spells and using magical items as a priest (Shadowdark, pg. 52).
SLAYER TALENTS
2d6 Effect
2 Learn one additional priest spell of any tier you already know
3-5 Your Slaying deals +1 dice of damage
6-9 +2 to Strength, Dexterity, or Wisdom stat
10-11 +1 to melee and ranged attacks
12 Choose a talent or +2 points to distribute to stats