dota only borrows models from warcraft 3. abaddon is arthas in warcraft 3. the makers of dota took arthas' model and remade it into abaddon.

WoW picks up from events after WC3:TFT. in the final part of the TFT campaign, arthas (who was a death knight, yes you can train them in the melee games) broke open the icecrown with frostmourne and put on nerzhul's (aka lich king) helm. in WC3 TFT, the lich king is trapped inside the block of ice in pieces of armor. you can see this block of ice (icecrown) in dota as well.

when arthas wore nerzhul's helm, he merged with the lich king. so that's why he is the lich king now. the lich in dota and in WC3 isnt related to the lich king except for Kel'Thuzad in WC3ROC and TFT's campaign.

so where was WoW based? answer is WC3, not dota. dota is just a bunch of random characters adopted from models/sounds from the original WC3 game. abaddon is an example.