Runestones are augments for Companions. Each Companion has three Runestone Slots which unlock at Lvl 1, Lvl 5 and Lvl 15 meant for Bonding Runestones along with Equipment slots which unlock at Lvl 1, Lvl 15 and Lvl 25 which can be slotted with Companion gear with their own Runestone slots which can give Offense or Defense bonuses.
Runestone
Effect
Companion Runestones
Bonding
Grants you Companion's Gift which applies 1/5/10/15/20/25/30/35/40/45/50/55/60/65/70% of your Companion's stats to yourself.
Your pet's powers that target foes deal 5.2/6/6.8/8/9.2/10/12/13% additional damage.
Serene
Defense
When your companion is struck by an attack your Companion has a 5% chance to gain 1.3/1.5/1.7/2/2.3/2.6/2.9/3.3% Hit Points.
Notes:
The lockbox runestones: Indomitable (Nightmare Lockbox) and Serene (Feywild Lockbox) come as a lesser of rank 8 and thus do not have a perfect rank. This is also confirmed via the Game Files. This is because the lesser stone is already rank 8 and rank 10 (greater) is the maximum. In module 6: Elemental Evil the maximum rank is raised to 12, however the new ranks are simply called ', Rank 11' and ', Rank 12'.
The image is for the lesser version, higher ranks add flair to this base look
Discarded Runestones
Major changes in module 16 have made some runestones ineffective.
Runestone Slots which unlock at Lvl 1, Lvl 5 and Lvl 15 meant for Bonding Runestones along with Equipment slots which unlock at Lvl 1, Lvl 15 and Lvl 25 which can be slotted with Companion gear with their own Runestone slots which can give Offense or Defense bonuses.
Bonding
Arcane
Empowered
Profane
Recondite
Indomitable
Serene
