you know desolator doesnt stack with your frost arow (or what ever was name of your first skill)?
You shouldn't leave Frost Arrow on autocast anyway, you should be orbwalking (individually cast it) but even given that, Desolator is a bad item on Drow. Manta Style would add a ton of survivability, Monkey King Bar would add more damage for only slightly more gold, and Butterfly would be best case. Between the need for orbwalking and the steep cost, Deso is just not a solid item.
The more concerning item is Soul Booster. I understand the need for survivability, but for only 600g more you could have gotten a Black King Bar which basically would buy you 4-10 seconds of magic immunity which would keep you alive for a pretty long time allowing you to dps like a boss, and adds some passive HP and Damage. The biggest problem with getting Soul Booster on physical dps carries is that you have nowhere to go with it. You spend 3300 Gold on an item that you can't really upgrade into something useful (Bloodstone is excellent on Semi-Carry Int heroes, but gives minimal benefit on Agi Carries).
items that add debuffs like desolator (-6 armor), eye of skady (slow) etc and life steal items like sacred relic, helm of dominator does not stack with each other and with skills that apply similar mechanics (slow, life steal...) with few exceptions
aura type skills/items are stackable vladimir's offering and leoric's life steal aura stack with items above however 2 vladimir's offerings in one team doesnt stack (not 100% sure but i think vladimir stack with leoric's aura)
mjollnir/maelstrom stack on certain condition which is: whenever mjollnir/maelstrom is triggered chain lighting suppresses all other effects from items and skills except aura type skills above.
edit: just to be clear doesnt stack special effects (-6 armor with life steal) but everything else stacks (+60dmg, 25str)
That's... mildly inaccurate. Certain "Orb Effects" or "Attack Modifiers" as they are called in DotA 2
do stack, it has nothing to do with them applying debuffs. They stack in funky ways. For example, Eye of Skadi on
all ranged heroes does not count as an Orb Effect. As you mentioned, Mjollnir/Maelstrom only counts as a modifier when it procs, so if it procs in 1/4 attacks and you had say a Satanic, you would steal life for the 3 attacks it didn't proc. On top of that, a lot of things mimic modifiers aren't actually--Skeleton Kings Lifesteal Aura does stack with Vlad's, Lifestealers Feast does stack with other instances of lifesteal because it's an "on-proc" ability not an orb effect.
On top of all that, there are weird instances like Luna's Glaives that interact oddly. The first target steals life, but everything hit by the bounces will not. Skadi and Diffusal Blade flat out don't work as far as I know. She will attack as normal, but never burn mana or slow.
TL;DR - It's complicated as fuck, but don't stack attack modifiers unless you're positive it works.
Here is a tactic me and my friend (Fernandres) have been using lately:
http://cloud-2.steampowered.com/ugc/1118299879203970157/00C2AEA3C7975892ADE396A9B0FF523B573E8D30/
We go rosh at lvl 1: I take Skeleton King with vampiric aura (dual stout shields, 1 pot), while ursa goes stout shield with pots/tangos, taking fury swipes. As SK, I tank the bulk, go only out to heal with pot, if needed(depends on the stuns), Ursa heals me a second time. Ursa should hit rosh continuously, the fury swipes debuff only stays 6s on him (nerf to prevent Ursa from kiting and soloing rosh at lvl 1). To make it easier, a third teammate can take Treant and protect SK for a few instances.
Here's what happens when that strat is detected ;
http://dotabuff.com/matches/137926405
i find the aegis of the immortal to be absolutely useless to a good team.
its only good for the losing team, gives their carry a chance to come back.
other than that.. meh.
if i have a good team who can handle themselves(which is rare), i go and kill roshan alone at lvl 14-16
else i dont bother
When you're playing at low levels that's true, but once you get to players who can actually co-ordinate, it's a game changer. Mostly Aegis is just a overly glorified support item on par for a Mek or Pipe.