Immovable is a very under-rated skill, while it offers so many advantages, especially when 2 skilled players are in a duel. You might loose 1-2 sec. to hold it up but it's the only skill against hard-CC and in pvp, atm, there're many so heavy-CC builds, especially NBs and Sorcs have in-build CC in so many skills, and players like to start with a heavy attack from stealth..
Imo, in the current meta (sorc&nb) it's very valuable, plus, the WB-spammers and all that templar skills with CC (Javelin, Sweep/Jabs, Blazing Spear, etc.) are "negated" by Immovable.
If you are only asking about the 'Stamina cost reduction' of breaking free, then yes, the cost reduction is based upon the number of Heavy armor pieces you are wearing... so if you're wearing ZERO heavy armor, then you receive ZERO cost reduction.
However, the skill itself can be used regardless of whatever armor you are wearing.
CP: 2078 ** ESO+ 2025 Content Pass ** ~~ ***** Strictly a solo PvE quester *****
~~Started Playing: May 2015 | Stopped Playing: July 2025~~