Here is a Dota 2 guide on when you should be buying a Null Talisman, or if you should at all, for different Intelligence heroes in Dota 2.
The Null Talisman is the Intelligence stat-based starter item in Dota 2, and there are some instances when you should consider investing into one – maybe even two. The Null Talisman is a great investment for certain heroes, especially those heading into the mid lane.
Null Talisman – bridging item gaps in Dota 2
Heroes that benefit from huge Int stat boosts early should almost always pick up a Null Talisman or two, like Invoker, Storm Spirit, Silencer, Lina, Void Spirit, and Queen of Pain. Their kits also highly benefit from the additional effects like spell amplification, especially Invoker.
As for the number of Null Talismans, two is considered the sweet spot. If you managed to win the lane handily, it might be skipping it altogether to get a higher-tiered item, although some heroes like to start with a Mantle or Circlet, both of which are components for the Talisman.
Like the Wraith Band, the Null Talisman has a primary component that does not build into anything else, which is the Mantle of Intelligence. Sometimes, it’s better to skip the Talisman altogether and invest into Circlet items and a Magic Wand instead.
The Urn of Shadows is a great pick-up for any mid-lane hero that will roam constantly, like Invoker. The stats it provides are decent, and you can certainly mark them as a replacement for the Null Talisman. If you’re aiming to boost your mana pool, Arcane Boots provides them in spades, but you will be missing out on the additional spell damage. However, since it can be disassembled, Arcane Boots is a better investment as the game draws on longer.