I was playing Diablo 3 and there is a certain monster, a cultist that just stands there, summoning a demon into themselves. If you don't kill them in a short amount of time, they burst into a nasty demonic entity that is hard hitting, HP, and - here's the kicker: FAST. They dash right into your face and start wailing. Any time these monsters appear on screen after you figure out what's up, it's a race to kill them fast, ignoring everything else in the room.
For the most part, I'd stat the transforming cultists as pure brutes with an encounter (or recharge) power that can be used as part of a charge. That simulates the speed of that initial rush when they first engage you. If you feel that they must be fast, give the demon form a base land speed of 8 or so, and you will be set.
If I were going for a totally literal translation, I'd have them start with standard Artillery AC and hit points and give them a multi-round transformation power that they activate to change into brute form. Assuming a level 10 monster, I'd probably use a set of powers like in the sblock.
[sblock="D3 Transforming Cultist Powers"]
Channel the Beast: (minor action) The cultist gains a charge of channel the beast. If a move or standard action is used, this charge can be donated to another cultist within two squares*. If the cultist takes any action on its turn other than using Channel the Beast, it loses all charges.
Demonic Form: (free action) When the cultist reaches ten** charges of channel the beast, Demonic Form activates until the end of the encounter. When Demonic Form activates, all conditions affecting the cultist end and the cultist gains sufficient temporary hit points to make the sum of its hit points and temporary hit points 126***. While Demonic Form is active, the cultist gains +3 to strength checks, +8 to athletics checks, and can only make Rending Slash and Brutal Slash attacks.
Rending Slash: (standard action, melee basic) Requires: Demonic Form; melee 1, +14 vs. AC, 2d8 + 13 damage.
Brutal Slash: (standard action, melee, recharge 5) Requires: Demonic Form; the cultist moves up to his speed and makes the following attack: melee 1, +14 vs. AC, 4d8 + 13 damage. This power can be used in place of a melee basic attack as part of a charge, but if so, the movement part of the power must still follow the rules for charging.
You might add some pushes or conditions to one or both of the slash attacks, and the recharge frequency was just a guess based on how often the demon cultists seem to do that big-damage charge. Notes for the choices are in the sblock. (Yay! Nested sblocks!)
[sblock="Asterisk notes"]* - Since the D3 cultist almost never transform all at the same time, I added the option of a cultist donating a charge to a nearby ally, giving them the option of having a staged/wave effect to the encounter. This also gives a preview to the PCs as to the transformative nature of the cultists. One more level of sblock shows an examples of encounter waves with three cultists.
[sblock="Example: Three Cultists"]There are three cultists (Alan, Bob, and Carl , in initiative order), plus a few other creatures -- say a couple soldiers and an artillery or a controller. The cultists all use their minors to build their own transformation, but all donate their move and minor to the cultist that goes second in initiative order (Bob). At the end of the first round, Alan and Carl have one charge, but Bob has seven.
On the second round, Alan donates his move and standard charges to Bob, building own as a minor. Bob starts his turn with nine charges. Depending on the range to the PCs, Bob can either transform with his minor then move to attack, or donate his move action channel, transform with his minor, and attack with Brutal Slash (or a charge action Brutal Slash for a "range" of 12 squares).
The key for the waved approach is to have the cultists donate all they can, making the next transforming cultist start with eight or nine charges.[/sblock]** - I picked ten charges for a three-round self-transformation. Well, three rounds and the first minor action of the fourth. Vary this up as you prefer. With the cooperative option above, it can be quite free-form as to how long it takes a given cultist to transform.
*** - The total of the cultist's hit points and temporary hit points is set to the level of a level 10 brute, about 126 hit points or so. This is to reflect that a partly wounded cultist still transforms to a full-strength demonic form.[/sblock][/sblock]To make the transformation more extreme, you could have their normal form be a lower-level creature, too (remembering to add back in the defenses when they transform, or even just having a separate stat block for the transformed form). Or you could make their normal form a standard creature and their demon form an elite.
Just throwing out some ideas to see what, if anything, sticks. Hope it's helpful.