Magic items are given sentience by trapping a psyche in them. This can be willing, like someone wanting to be part of an instrument of good rather than being sent to a Soul Gathering System, or it can be forced upon them.
Taking ownership of a sentient item is risky, especially if it has a malevolent psyche. At some point, there will be a contest of wills, item against owner. Even if the owner is stronger, there may come a time when they're weakened physically or mentally, draining their willpower. A series of wounds, deep illness, or having their mental faculties wrecked by a mind attack can lead to the sentient item becoming master, dominating their owner, making them their puppet.
Grarg's thoughts are his own, unreadable by Rax. This is not so with Rax's thoughts. As long as Rax doesn't deliberately block the telepathic link, the sword can read his thoughts as if they were its own. It's that way with all telepathic magic items. Some won't own such items, because they never have the solace of being alone, while others like them because they never have to be alone.
- from the Godspawn Saga