There are a few fundamental abilities to look for when selecting a commander for a lifegain-themed deck
Plenty of cards from that set offer benefits based on the number of party members or even having a full party with one of each creature type
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Massacre Wurm peaked at $30 and settled at $25
Jaheira, Friend of the Forest turns all of your tokens into mana dorks Treasure tokens included allowing you access to large amounts of green mana, pretty much how a strong druid in Dungeons & Dragons would in a similar context
Finally, it has the Welcome Return ability, which states that whenever this character quests, you may return a chosen character of yours with a cost of one to your hand (from your discard pile), and if you do, you may play a character with a cost of one ink for free