What makes Spirit bear special, compared to other summons and heroes? Unlike summons, it can carry items; unlike heroes, it does not benefit from stats (str/int/vit) on items. Considering that any form of Regeneration is pointless, the full list of viable items is really short: boots/ Phase boots, Orb of Venom, Stout Shield, Radiance, Hyperstone, Maelstrom, Mjollnir, Assault Cuirass, Basher, MKB, Buriza. Boots+OoV+SS are the standars basic items on Spirit bear. Other viable items are expencive and thus require careful consideration.
Radiance is the most classic core on Syllabear. If completed early, Radiance increases Spirit Bear's battle presence tremendously. But farming for 3800 gold expensive Sacred Relic always instigates enemies ganks and pushes, which can result in late Radi and lost game.
All other items form two groups: damage and attack speed. As AS is more preferable for better Entangle, Hyperstone and AC are possible extensions. But, while good for push, AC is not good enough in fights.
Mjollnir gives both IAS and Damage. Chain Lightning strikes multiple targets, Static Charge damages eneimes that attack target or damage it with AOE, in other words, it's the best item on Spirit Bear. There were graphs on forums that it's one of the best DPS items on heroes. But it's bad for Entangle, isn't it?
Entangle and Chain Lightning
Because of being Orb Effect, it's not a good item in Warcraft DotA (it stopped Entangle procs completely). In Dota 2, it reduces Entangle proc chance to 15%.
1. Frequency of Entangle procs.
Spirit Bear lvl 4 with Rabid lvl 4 and no items (or Radi) attacks once per second. Thus the mean time attacking utill Entangle is 5 seconds.
Spirit Bear with Mjollnir attacks faster, even with reduced chance it improves mean time to Entangle to 4 seconds.
2. Super IAS: Worth or not?
In the description of Entangle, there is a 5 seconds cooldown. Even is Spirit Bear's IAS is so high (250 IAS, feasible with 4 Hyperstones) that without CD Entangle can proc every 2 seconds at the mean, with this cooldown it will proc once every seven seconds.
If one of those 4 Hyperstones was Mjollnir, it would increase the mean time of attack to entangle to 2.5 seconds, and the mean interval between 2 entangles to 7.5 seconds.
7 vs 7.5 ... or it could be 9 vs 10, or it could be 8 vs 8 plus something.
Is this a big difference? No, it's not.
When Mjollnir is more than Radi replacement?
Sometimes, your enemy is a carry such as Lycan, of Ursa, or Antimage, or Morfling. They don't care if you have Radiance! They kill your supports, they kill yourself, and they don't give a fuck! There were games in International lost because of this!
Well, with Mjollnir bear, nobody can say that! The DPS is 50% bigger, lightning is bouncing around, and your next item is gonna be Basher! Not mentioning that you don't sit there like a pussy(you're a bear, goddamit!) farming 3800. Fight before you get Maelstrom, fight with Maelstorm, destroy the ancient with Mjollnir!