The short answer is yes.
Here is the description to the Backstab skill from Rikimaru
If Riki attacks from behind, bonus damage is applied based on his current agility.
That said, all attacks from behind will do extra damage that can be calculated (ignoring armor resistance) by adding the ammount added to the base damage from your character.
Here is some information about how Critical Strikes work:
Critical damage is physical damage and reduced by various types of armor. The red number shown by critical strike is displayed before reductions (this is why illusions don't show very low red numbers). Critical strike does not work against buildings and is subject to pseudo-random distribution. Because critical strike multiplies the attack damage of a unit, it is a scaling ability (unlike the damage of Monkey King Bar's mini-bash ability).
Backstab and critical strike can proc in the same attack. Period.
What's not clear here is when will the critical strike bonus be calculated.
The backstab skill adds bonus damage to your atacks based on your agility, and critical strikes have a chance of x to multiply by y your atack damage.
Lets say critical strike damage is the last parameter to be calculated. That would lead us to something like this ( I picked the values )
Attack Base : 50 , Backstab Bonus : 40 , Critical Strike Multipler : 2
Total Damage = (50 + 40) * 2 = 180
On the other hand, if Backstab bonus is added after the Critical Strike sums up, we would end up having :
Attack Base : 50 , Backstab Bonus : 40 , Critical Strike Multipler : 2
Total Damage = (50*2) + 40 = 140
TL:DR
Whether the Critical Strike is calculated after or before Backstab sums up, is not clear, but they CAN proc in the same attack.
BristleBack is vulnerable to a lot of things :
- In early game he is fairly easy to gank and burst down with magic damage. He has no escape mechanism and the skill birstleback is at low level. Someone like Gyro can take him down solo (if Gyro is easylane vs bristle hardlane). If Bristle is the ennemy safe farmer, let it be, he won't be able to carry as hard as PA, Gyro, Phantom Lancer etc.
- BristleBack is not a good farmer, his killing potential come from his skills and stacking charges of his ultimate. Burn his mana and he won't be able to do anything (Diffusale Blade, Lion or KotL are usually pretty effective)
- BrislteBack has no disable whatsoever (and rarely tend to farm one), carry a TP scroll on yourself and prevent any further gank.
- Basically don't try to chase him, let him leave even with low HP. BristleBack's player rely on taking damage to trigger the passive and stack quills spray faster : no chase > no quill spray > no useless death.
- If BristleBack is picked, dont pick a melee carry that will have an horrible time versus him in the laning stage. If Bristleback is counterpicked to your carry, concider trilaning with 2 strong support (Crystal Maiden, Venomancer, Keeper Of The Light) in order to harass him out of the lane or get a good healer (Dazzle can save you with shallow grave and negate the quill spray damage with shadow wave).
- A good melee counterpick however is Legion Commander as Duel force Bristle to face LC, (Same goes with Axe)
- Watch your Quill Spray debuff if laning against him (as you would take care versus Batrider's Sticky Napalm) and let the debuff goes when there are to much stacked.
- At last : buy a magic stick as soon as possible, you'll get a lot of charges and it can save your life !
And for the love of god if you have Rikimaru, repick him immediately as you can consider your game lost versus a good Bristle.
To finish : Having 2 melees carries (Phantom Assassin + Nai'x) that both rely on farm to be effective in the same lane is generally a bad idea. You'd better get a support or put Nai'x in the jungle.
Best Answer
There is no limit to the number of charges an urn of shadows can get. But there should be no reason for you to save too many charges since they don't provide an additional benefit. Also, remember that the heal/damage is a fixed value so the later the game goes, the less useful it is.
Also, the radius to cast the urn is 950 while the radius where it gains a charge is 1400.