You do get the increase, it just doesn't show as a buff. After he kills someone, his attack rating and hence his damage increases for all further attacks in that round / arena battle. 

Nebios said:

The invisible - Fabled Skill buff - is for the battle? Not only until the Mountain King has the next turn? I understand! Thanks for the help. 

I think it works like most boosts of this nature, which is that it's reset each time between the first, second and third part of a dungeon or campaign battle. In arena, it's just for the duration of one battle, yes. 

As the description says, you can even have two boosts like that, effectively doubling your attack rating, if you've killed two champions. 

I got Mountain King like 6 months ago but never touched him till Rotos nerf, I have 2 Rotos then replace 1 by MK. 

MK is really really strong, in case you don't build him strong enough, consider this is a help. (if you are "expert", who got a champ that kill all enemies, just ignore it)

MK attack are [HP][ATK], I focus on building hp% + atk%, build him very high hp and high crit damage, also make sure total crit rate is 100%. You may have to sacrifice other stats, no acc, no resist, no def, no speed, in that order. 

Attack: 1st choice is A2, save A3 for special case, first enemy choice is blue, void and red. You could use a blue to kill green enemy, in my case is Rotos. 

When you use A3 ? Use it on their most important champ, like Siphi, Serris, Krisk, etc. and often use it on 2nd or 3rd round, when he/she got a hit like 50% and shielded up, you use A3 to kill him/her even with shield on. That is the power of Mountain King.