This buff exists only on progression servers and from Classic to PoP. It's lifted in LDoN era for LDoN bosses, but remains on raid bosses or mobs from previous eras once your server has entered LDoN. There is a debate whether it should be totally removed, but that's another topic.
The buff was intended to balance the strength of returning players by making bosses harder on progression servers. The main effect is that the protected mob will mitigate incoming melee and non-melee damage for a fraction which decreases era after era.
- In Classic (to be confirmed) and Kunark, the fraction will be 40%.
- In Velious, 30%.
- In Luclin, 20%.
- In PoP, 10%.
Players and pets will also be affected by the counterpart of Mitigation of the Mighty, which is called Presence of the Mighty and which decreases your resists and increases incoming spell damage.
However, there is another effect which directly impacts petclasses. Any mob with MotM will apply a special aggro rule making it impossible for it to target a pet. If the pet is first on hate list, the owner is automatically targeted. Get it ? Very, very, dangerous... And no mage/necro/bst expeditions to kill older raid content, unless a bst tanks ! Do not use epic pet on MotM mobs ! Anyhow, with this penalty, once in Kunark era and till Luclin era, your best bet is to stick to fire pet both for dps and for aggro purposes.
This special aggro rule was implemented to avoid pet walling of good old dragons like Trakanon when servers like Lockjaw were in Kunark era and a guild petwalled Trak which became the big scandal in 2015. Go figure, the heated debates two years ago seem so far away and ridiculous now... Tempus fugit...
NB - to further penalize the use of pets on raid bosses, another pet specific debuff was created back in 2015 "Mark of the Old Ways" which made pet melee and non melee weaker the more pets were on the same mob. But this has been lifted ; just a short reminder here.