That's actually exactly what I run. The Eversor plays a very important role in the whole set up. He uses his 18" charge to get into combat potentially even on the first turn, creates a LOS blocking clump and allows the other 3 assassins to move in for the kill with much less to worry about.
The combo is quite nasty and most opponents aren't quite sure what to do about it. I've been running it for a while and the most typical reactions seem to be
Do basically nothing and focus on the rest of the army: One of the guys that did this lost about half his force when they started breaking off the table or getting rundown after loosing combat by int5 assassins. Remember seperate units so you get 3 seperate tries to catch them. You can also use a single assassin standing out of combat to invoke the "no rally within 6" of an enemy" rule.
OMG Assassins!: This is by far the more common reaction. I've had instances where my opponents entire army was manuvered and fired as if the rest of my force had disappeared and the assassins were all that was left. This is also in a large part thanks to the Eversor. Even though in a typical game the deathcultists will cause far more damage than the Eversor the Eversor's the one that makes opponents neverous when he infiltrates 12" away from their pricey devestator squad.
Ok now for the benefit of all my new converts here here's a basic list of assassin do's and don't I've come up with during my tour as an assassin master. These apply mostly to Deathcult assassins but they carry over pretty well to the Eversor as well.
DO
-Remember as a deathcultist or an Eversor that you have +1 attack for 2 CCW in addition to what's listed on your profile.
-Deploy your assassins in a group or at least a pair: One assassin is typically too fragile to attack a unit on her own without being struck down by counter attacks, 3 will decimate virtually any unit typically clearing the kill zone with wounds left over.
-Spread your assassins just right when you attack: Ideally you want to clear the enemy kill zone of counter attacks and not have any spillover wounds that don't kill anybody.
-Know your target type: Assassins while pretty cheap for what they give you are not so cheap that attacking grots or conscripts is a good idea. If possible you want to look for targets that are high cost individual models reasonably suceptable to S4 or are going to be so utterly helpless against you that you can use three assassins to tie up three seperate units of them. Guard heavy weapon teams are a good example of the latter. Most importantly though pick targets that you swing first on. A big part of what makes DC good is the int 5. It compensates for their fragility if you can kill all or most of what's going to be attacking you before it swings.
-Kill IC characters and uber combat units that will deciamte the rest of your army: This is probably the singularly most fun use and a great your cost VS his cost pay off. A lot of hardcore combat units like assault termies/marines lack the int to strike at the same time as assassins much less before and often times also the WS to avoid being hit on 3s by their WS of 5. This means you can slap the crap out of them and probably destroy their unit over a few turns while keeping your team largely intact. Don't be afraid of int5 units or characters either. They'll typically cost way more than the 120 points it's going to cost you to slaughter them. How many ICs out there do you know of that will win reliably against something with 6 wounds a 5+ inv and 12 PW attacks on the charge or 9 standing still. He's also not going to b hitting assassins on 3s like he's used to. I've killed Grand masters with termie retinues before and they were insta killing me so believe me this works.
-Try to stay in combat as much a possible: This is just basic assault troop tactica. The only time you shouldn't be trying to get in combat at all costs with your whipeout/consolidate is if you have reason to believe the opponet will ignore the assassin in favour of something else, there are no guns trained on you at the moment, it's your turn next=free charge, or you want to create a choice for the opponet to shoot at your assassin or something else important.
-Sew terror and chaos: I use this stragety quite often myself. Rather than being particularly picky about targets I just rush in and start killing/drawing fire. The disruptive effect this can have on an enemy army is shocking, especially when we're talking about armies like guard or min maxed marines where a single unit is unlikly to be be able to defeat a single assassin in a single turn, if ever.
-Attack a flank or (reletively) isolated area: This is a tricky balence to strike because one of the best parts of assassins is the distraction they cause but at the same time charging them right into the middle is pretty stupid. Attacking a flank is preferable for several reasons.
-It protects you from exsessive firepower while still allowing enough potential LOS to make the assassins a tempting alternate target for certain units.
-It's easier to roll up a rug from one end and if you can kill yourself a safe zone you can use whatever assassins you have left to start eating your way across the army.
-Reinforcments/counterchargers he sends to stop your assassin rampage will typically end up in the ass end of nowhere even if they do kill your assassins. This typically opens up nice big holes in his counterattack ability elseware which enables you to unload flamer squads and torch all the unprotected linetroops without fear of getting smoked by assault terminators or something.
Basically there's two ideal ways to use assassins
One is to never bite of more than you can chew at any one time. By this I mean you work the various assassins as a team, or teams if you really went nuts, in isolated areas and gang up on an enemy unit or two startng early in the game. Keep your assassins together and just hop unit to unit killing things. How many units you engage at any one time will be determined by what those units are, keeping in mind you want to minimize counter attacks as your top priority. You pretty much need to get a feel for this method but once you get it down and start looking at the amount of VPs your assassins got you after the game you'll be shocked and impressed. My current record lays somwhere in the 6-700 range with a combination of kills and run downs using 3 cults and an eversor which is about 3.5 times what they cost to get.
The other way is to just go for maximum disruption in a given turn. You basically say "gee I could really use it if he shot at me less for the next few turns" and then you spring your assassin squad when you want that to happen and try to attack/distract as many units as possible both just by showing yourself as a target and by charging the guys with the scariest guns.
The ideal master of assassins will combine both of these strategies, and others, as the pulse of the game changes.
DON'T
-Make your team the only viable shooting target for several enemy units that carry significant firepower: Guess what happens when you do.
-Charge units in cover, especially assault units: You don't have the T or the save to ride out a bunch of strikes first attacks with assassins. Only charge a unit in cover if there's no other option and if you do, when you do, only charge one side/corner of it to keep the attacks to the minimum this turn. You can step in and kill them next turn when
you're striking first.
-Bunch up, you're not a real squad: Nothing is going to make you feel stupider than to loose all 3 or more assassins to a pie plate when there was absolultely no reason why that had to occur save you pretending you needed a 2" cohearency.
-Spring them too early: There's no reason why you have to make your move on turn 1, 2 or even 3. Sit back, move from cover to cover, make your opponent very nervous. That's not to say you shouldn't spring them early ever, if you think they're going to do some real good do it, just don't feel pressured. Even if you go Eversor 3xDc like I do that's still only 215 points out of your army so it's not like you have to start climing "make my points back" mountain right away. Sometimes timing them to hit just before or at the same time as your main attack is much more valuable than an early distraction your opponent can potentially recover from.
-Charge into a horde unit: There's no quicker way to waste a squad of assassins than to try and throw them at something like hormogaunts, mutants or choppa boys. Anything with a high count of cheap models and a grotesque number of attacks is something you want to avoid. Assassins are assassins not lawnmowers, you've got flamers, heavy bolters and mass boltguns to deal with
this type of threat. The only time I'd use assassins against a horde would be with the support of a troop unit like sisters ot zealots. You can not absorb massed attacks with T3 and a 5+.
Ok that does it for the main points that come to mind right now. Keep in mind that these are all just rough guidelines and observations based on my personal experiences with the unit. In other words your milage may vary. The best way to learn assassins is just to include them in your army and commit to not taking them out for at least your next half dozen or so games. Try them against varied foes and maybe tweek the rest of your list just a little to take advantage of what they can offer you. Most imporantly just have fun and don't ever forget to point out to your opponent how the model that just tore the head off his uber commander or killed half his terminator squad costs 40 points.